I couldn't agree more with jitteryevil's guide. I bought one for 2 1/2 yr old, and he filled it, although again not large.
I bought it because I spend a lot of time on public transport. He looked so uncomfortable when asleep, although I purchased a neck cushion which sits either side of the head to support him and it did keep his head up in a more comfortable position.
I also sold mine because it was too flimsy and it was a pain to fold / unfold it and he prefers to lie down when asleep, even now at 4, as he has mobility problems and still needs a buggy.