Sorry, Stephanie, I didn't see the question.

This is only my second year of growing OG's from seed, so with the possible exception of some cool season grasses, I don't think I have mature grasses yet.

Chasmanthium latifolium reached 2.5' it's first year, and 3' this year. It's approximately doubled it's girth.

Miscanthus sinensis 'New Hybrids' reached 4' to the plumes in the first year, and 5' this year. It's also doubled it's girth.

Pennisetum alopecuroides 2' first year, 2.5' this year. ~50% increase in girth.

Panicum virgatum 2.5' first year, 3' this year, with only a slight increase in girth.

Deschampsia caespitosa 1' first year, 2' this year, and more than double the girth. This is a gorgeous spiky ball of grass. I'll start a lot of it next year.

Luzula nivea, 1.5' first year, 2' second year, doubled it's girth. Another nice grass. I started another dozen clumps this year.

I can't speak from experience on 'Hameln' but the books say it's mature height is 3'. I assume that's to the plumes. They also say that it doesn't self-seed, which could mean it never produces ripe seed, or it's seed isn't viable. An easy way to test the seed is to wait until late fall to harvest it, and give it the damp paper towel in a baggie test.

Regarding division of the gallon-sized plants. If it were me, I'd wait until I'd grown it for at least a year. This way, you get an idea of how it'll do in your garden, before making the decision.

My dwarf fountain grasses grew every bit of 3'x 3'. Leaving the seed heads up resulted in self seeding also. It even grew out of expansion joints in my patio underneath my awning. I have seen the regular fountain grass self seed into lawns, and into small junipers. I still like the plant, but am more picky about where I plant it.

I agree with lol, let it grow some roots before dividing it.