Funny I was going to come on here and ask something similar, so I thought I'd tag on here.
We have just got back from our hols with Tilly and seen *loads* of cocker spaniels, but one of the things I was struck by was how though most of the dogs we saw had more bulk (muscle/fat) than Tilly, but at not quite 5 months she wasn't much shorter, she only looked smaller because she is so skinny right now - no idea what she weighs at the moment.
I know the muscle tends to come later and they fill out, but I wondered how long they grow height wise? To give you an idea how big she is, when she sits in the smaller crate we got her at first (24x17x19) there isn't more than an inch or two clearance between her and her head - she can turn round, but she pretty much fills it and much prefers the next size up crate we bought her! I seriously doubt when she fills out the smaller crate will be suitable to use!