Are there any standard clothing restrictions when visiting mosques?

Clothing restrictions in mosques often vary from country-to-country, and even within mosques in a country. Everyone is asked to take their shoes off at an entrance area. For men, no shorts are allowed; for women, no skirts or bare shoulders, in addition to this some countries also mandate women to wear a scarf.

Those are the basics, but how 'welcoming' a mosque is to tourists depends usually on how many tourists visit a place. The more frequented ones often have robes that they give you for free to 'cover yourself up' in case your clothing is deemed inappropriate. Smaller and/or less frequented mosques may not have such provisions.

Off-topic: Mosques often have special entrances for non-Muslim visitors. Don't go barging in through the main entrance; if unsure, ask. Most mosques also do not allow visitors into the main prayer area. Having said that, most people - or staff, if the mosque has them - are friendly and will help you out with trying to resolve clothing restrictions or other questions.

Generally it would be said to you if you need to do some specific - rules are various from country to country and even from town to town.
I can't remember something applied to all except that you really should not eat or sleep there, or something like that :)
Dogs are banned too.
Calm, peaceful and polite non-Muslim will not attract negative energy during visiting the mosque.

It does depend on the specific country and the norms of the place. In the Putrajaya Mosque in Malaysia you get given a pink robe to cover up, and even then you can't enter the mosque proper if you're not Muslim; yet while visiting the Netherlands with a youth travel group of all sorts of backgrounds (and attire) we were welcomed into a Turkish surau (a mini-mosque of sorts) as we are.

Loose and modest tends to be the way to go really.

to be more accurate.

For Men it's like the churches.

you required to cover from shoulders to knees.

**Short are accepted if they cover the knees as most scholar says. (I pray with short in Saudi Arabia all the time), but some mosques are banned shorts, not all of them.

For Women:

If they need to pray:

They need to cover everything but not her face and hands.

If they need to visit only:

it's different and less strict and different from country to another :). and for a general role she need tall pants and t-shirt at least.(this is the basic in all places) in Saudi she even need to cover her hair(this is the Max).

**pets are not allowed especially dogs, so be careful.