when i have an object with more then one mesh e.g. a car with separate bumpers or a gun with removable scope or something, would it be better to make a small separate texture for each part or one big texture for all or doesnt it matter at all ?
Anything that can use the same shader and material settings, use the same texture.
Read the section called Don’t Use Many Materials. It applies to everything, not just characters.
http://unity3d.com/support/documentation/Manual/Modeling%20Optimized%20Characters.html
You could get a performance boost from atlasing a lot of other objects in with your car textures, but this can get unwieldy quickly and doesn’t lend itself well to changes. There’s a balance to be struck and you’ll have to find it for your own projects.
thanks, shame on me i didnt take a look at the docs