As long as your brew has head through the entire glass, size doesn't matter. I'm enjoying a glass right now that started off with about 1-1/2" of head, which has settled (at about 1/2 glass left) to a fine layer.
IF you want to retain good head, have clean glasses (well rinsed) and brew it for better retention. What you include in the recipe impacts more than just flavors. Your processes can also impact head retention. Such as if there was any soap residue in your pot/kettle before you started the batch.
I made a batch on Saturday that is the first where I've NOT used fermcap in the boil, or fermenter, in some time. I'm interested in seeing if that has any impact, at all, on the head retention of my brews. I don't think I added it to the primary for my previous batch, but I can't recall exactly (it's been there for just over a month).
Post up a recipe that you're not happy with the head of so that we get a better idea of what you're brewing...