That's really a difficult question. Some cattleyas need to get pretty large before they'll bloom, while others can bloom when they're surprisingly small. You really need to know the reputation of the particular plant in order to determine what size plant will possibly bloom.
Unfortunately, as the others have said, there isn't a true "definite" size. What I would do is look at the ones you are interested in and then call Hausermann's and ask them. Whenever I have done this, they have been more than helpful. If you are able to go to Hausermann's nursery, that would be even better!