i agree w/ pps. my LO eats a ton compared to other kids and i let him eat all he wants until he is finished. we do have those days when he eats a few bites or doesn't want what is served him and then he hands me his plate/utensil and puts his arms in the air for "all done". i try NOT to say "one more bite", etc b/c he knows when he is full or not hungry and i don't want to force him.
however, i understand your concern re: communication. my 16 mo is getting better w/ talking but we use some sign language, particularly at meal times which is incredibly helpful. he also picks up new signs very quickly. he signs "eat", "drink", "more", "all done", "banana", etc. i know it seems crazy to start now but it is SO helpful. we had a lot of whining at mealtimes that drove me crazy and that has stopped.
as far as general portions, there is a sticky in this section that discusses portion sizes. also, you could always offer a small amount and if he eats most of it, then cont to give him more until he stops eating.