How strict is United Airlines First Class with carry-on luggage size?

How strict is United Airlines First class with carry-on luggage size?

Not at all.

The published carry-on limit is 9"x14"x22" and the actual sizer¹ dimensions are 10"x15"x23". For personal items, the published is 9"x10"x17" where the actual sizer dimensions are 9"x11"x18".

However an airline knows which side of the bread is buttered and unless you try to carry on something truly atrocious, no one is going to pester first class customers with the sizer. Air Canada at one time measured the weight of the carry on bags even at business check in, that lasted about three months before they realized that's not good for, ahem, business.

¹A sizer is a physical frame, usually located at the gate, that you/they can slide the bag into to check its size in case of issues.

Given that it fits, you have nothing to be worried about. You are absolutely required to have your passport with you and highly recommended to have your valuables with you, plus medicines and other essentials, so it is normal to use a carry-on or personal item (or both) to have these. Most laptops even fit in personal item sizes which are more restrictive than carry-on.

United is not strict at all about carry-on sizes and I have seen countless clearly oversized bags being taken into various short to long flights with them. On small planes they offer to take larger ones as gate checked luggage to those who have among the largest items, most typically roller bags.

While they do not actively measure carry-on size, there is a sizer at most gate entrances. This is essentially a metal cage the maximum size of the carry-on they allow. If your bag is suspected of being too large, you can be asked to place it in the cage. If it fits, then you are good to go. If it is not, then they can ask to convert it to checked luggage, in which case you may be charged an extra fee. You will usually be instructed to take out any valuable items and essentials.

A personal item is a good way of ensuring you will have your most important items with you and United, as the majority but not all airlines, allow passengers to bring one in addition to your carry-on. You can use pretty much anything as a personal item, even a folder as you suggested but I have been plastic and cloth bags as well as unpackaged items. It's a good idea to carry an extra cloth bag inside your carry-on that you can use to carry essentials in tight planes where even an allowed carry-on does not fit in some overhead bins and you end up in a seat without space under the seat in front of you or no seat in front of you at all.

There are two issues that need to be considered :

1. Whether the staff will let you board with your bag, and
2. Whether there is sufficient space for the bag once you do board.

The first of these you have basically answered yourself - if it is within the size allowed, then you will be fine. United Airlines do not consider the weight of carry-on bags, and you've already stated you are within the size guidelines.

To your specific question around how strict they are with sizes, the answer answer is "it depends". In general they are not overly strict - especially for passengers boarding in the earlier "boarding groups", which includes First Class passengers. On occasion a specific gate agent will decide to be more strict, including requiring all passengers with bags that look even close to the allowed size to check their bag against the "sizer" to make sure it is within the requirements - however this is rare.

The second issue is one of space on the plane. Even if your bag is within the size requirements, if the plane runs out of space for bags in the cabin then you may be forced to "gate check" the bag (ie, they will take the bag and treat it as checked luggag). Whilst it is common for this to occur (especially on a full or near full flight) you can avoid this occurring by boarding the plane earlier. By virtue of being in First Class, you will be in "Boarding Group 1" which means you will be allowed board relatively early - as long as you board at that time you will have no issue with space.

If you are forced to check the bag for any reason then you will be allowed remove anything you need from it and carry them onto the plane separately. Obviously that's still not ideal, but it means that you can remove your passport/laptop/etc from the bags and carry them with you.

At most airports, carry-on size is gauged at the security process, which no single airline has any control over. I've read of cases where airlines have advertised more generous carry-on sizes, for their passengers to find themselves facing issues at security.

Once you're past that stage, the worse that's likely to happen is that as you board, you'll be told that overhead storage is limited and you'll need to "gate-check" the bag you were hoping to put overhead. I presume that this is less likely in the front-of-the-bus classes than where I normally sit.

I'm an infrequent traveler and hardly in the position of offering sage travel advice, but I'd keep my passport in a pocket, because losing it would be such a pain. I tend to make sure I have a shirt pocket I can load up for things I might want to handle while I'm strapped in (boarding pass to resolve set assignment questions; phone; a pen) since it can be difficult to get to even your under-seat storage during turbulence, or when your tray table is down, or just if the seat next to you is occupied.

I've got a fair amount of experience traveling internationally and am keenly aware of the restrictions imposed by different airlines. Unless one is on a economy fare that most U.S. airlines are now offering, you can bring a carry on bag that fits in the overhead compartment and a "personal item" the must fit under the seat. I have a small pack that holds my laptop, a tablet, head phones, and anything I might want immediate access to during a flight. I have a few medications that I need that go in there as well. I don't mind gate checking a carry-on but don't want to gate-check a bag with my laptop and other "essentials".

I also have a small travel wallet with a cord that goes around my neck. It holds my passport, boarding passes, a pen (for filling out customs forms), and some cash (both U.S. and, if I have some, the currency at my destination). I wear it around my neck until I board, then put it into my personal item, and remove it before de-planing. I've never had an issue carrying it with a carry-on bag and a personal item.

I have seen airlines check carry-on sizes, mostly when there is a full flight, but usually they ask for volunteers to gate check larger carry-ons either to be picked up just outside the plane, or someones checked through to a final destination (for free).