What unusual requirements are placed on baggage when flying on a very small aircraft?

I used to fly on Twin Otters regularly. What happened was you walked out to the plane and handed your carry-on, such as a brief case, to the copilot. They put it in a compartment at the rear of the plane. Then you board via a few steps.

It was be possible to take something aboard, but there was no overhead compartment so this left only under the seat in front.

As the flights are short and scenic on these types of planes, it really didn't matter.

The aircraft shown in the picture is a Cessna 402 flown by Cape Air. Their official baggage policy for such aircraft is similar to WW.'s experience; I've emphasized the pertinent text in the quote below. There also appears to be a hard upper limit on the weight of "overweight" bags, as well as limitations on oversized items (i.e., don't bring 'em.)

For each ticketed customer, Cape Air will transport:

1 checked bag not to exceed 50 lbs or 62 linear inches plus 1 gate checked bag not to exceed 45 linear inches plus 1 personal item (e.g. small purse, briefcase, laptop) not to exceed 36 linear inches. These aircraft do not contain storage space under the seat or overhead in which to safely stow in-cabin items. Accordingly, personal, or carry-out, items must be carried to the aircraft and stowed by a Cape Air agent into the designated baggage compartments prior to boarding. ...

For each ticketed customer, Cape Air will transport, subject to available space and additional fees up to two (2) additional bags per passenger. ... All excess baggage is carried on a space available basis and is subject to an additional charge.

Baggage compartment constraints of the aircraft may restrict the weight or the size of the individual bags that may be accepted for travel. Baggage that is not accepted for travel pursuant to this section is as follows:

• Individual bags that exceed 70 lbs in weight or that exceed 62 linear inches in size. ...

• Bicycles, surfboards, canoes, kayaks and other such large pieces of sporting equipment exceed the capacity of the compartments and are not accepted for transport. ...

It is not permissible to transport cabin baggage in any aircraft by strapping the item to a passenger seat.

Boutique Air in California flies PC-12 single-engine turboprops about the size of a Cessna Caravan (photo from Wikipedia).

Their baggage policy currently is as follows:

Note that two of the airports they serve do not allow luggage to be stored for a later flight so if they run out of space you might have an issue.

Most people on these flights are not carrying huge piles of luggage. I carried on a fairly fat backpack with computer inside (four flight segments total) and had no issues (but was not allowed to put it in the aisle). Definitely a civilized alternative to the other option I had for LAX<->Oakland (Spirit Air). The only unfortunate thing is that you have to go through the same TSA nightmare as everyone else.