What is the dresscode for non muslim men? I read somewhere that the lenght of the sleeves are important.

Dress code is the same for all men, and you will even see Saudi men wearing western clothes.

In particular, your legs should be covered to your knees, and your shoulders should be covered. Thus, t-shirts, and short sleeved shirts are acceptable. Bare chest, and small shorts are not accepted.

If you have any more questions please ask. Also look at something like Saudipedia (www.saudipedia.com) for practical information.

normal shorts and cut offs are ok on the beach or by the pool... inside the western compounds anything goes... you should be fine and won't feel out of place... half the people here are not saudi so most wear pants and shirts... you should be fine...

I assume you are referring to outside the compound.
Let me start by saying this...

Cultural dress can vary between Middle Eastern countries. One can see a man from Morroco walking the streets wearing a muscle shirt and shorts while a man from Saudi Arabia might only go out in public wearing pants and a long-sleeve shirt. It's mostly just cultural customs.
Islamic law only states that the minimum standard a Muslim man must cover is from the navel to his knee (including around other men).

So if you were in a health club in Jeddah,
you would not see many men naked in the locker room. Keeping a towel on would be recommended to avoid offending anyone.

In the streets,
you won't see many men in shirts with no sleeves (muscle shirts/tank tops). T-shirts are very common and are just fine (as others have suggested).

In regard to wearing shorts,
you will see younger boys with long shorts, but it is not as common among men. While you most likely won't be harassed by wearing long shorts, you will feel more comfortable with pants. My husband finds drawstring pants to be the best for the heat.

Thank you all for your replays.... they are very informative and now I have a pretty good picture. I did not want to be embarrased or embarrasing anyone else.