I think both of the parks are relatively easy to naviagte. IOA is pretty easy since it's one big cirle. The new Harry Potter construction makes it a little tricky, but I believe there are signs/TMs to help you naviage thru.
US is a little tricker than IOA since it's not a big circle. If you just stick to the perimeter of the park you'll hit all the attractions. They have some cut-through paths that make it easy to get from one side to the other.
The parks aren't small, but they aren't huge either. Probably a little bigger than Hollywood Studios.
Have fun on your trip!