Donna, just looking straight at a totally unfamiliar name, I would be harder to guess what size the plant is or will be.
But then, looking at the parentage of Cattleyas - if I see something with Guarianthe bowringiana, I might expect a tall, good size Orchid. Or on the other hand looking at a primary hybrid with C. walkeriana I might be thinking window sill or something like that. Most Rlc are good sized too. And anything with Grammatophyllum will take over your house!
Most Pleurothallids stay at a manageable size, while there are many Cymbidiums (not all) which get to be huge.
So I would say it is a mix of experiences, simply having seen a lot pf Orchids.
