Guessing Age - Eventual Size

I'm considering a kitten right now and trying to figure out the age, as well as how big it may get.  With dogs, I've always found it relatively easy. Being new to cats, from what I understand, it's a bit harder.  Does anyone have some ideas as to getting an idea as to the age and eventual size of a kitten?

Thanks.

I go by the feet for kittens, just like a puppy! Big feet mean a big cat. As to guessing an age, there are sites that show actual pictures here of every age, an adviser ought to be able to help you, or look under the forum 'Pregnant Cats and Kittens' I can't wait to see what others suggest!