You have the small cell foundation and place on it you average run of the mill large celled bees. Would they be able to build from the small cell foundation? I'm guessing they would. Why not? Surely they can chew out what they need to and get on with building what would be comfortable for them.
They will build about 5.1mm as has been said. Why not ask a better question? If you put them on nothing what would they build? If the spaced the combs apart like they wanted and built the cells like they wanted? http://incolor.inetnebr.com/bush/images/BroodNestInFeeder.JPG
First they would build brood combs as close as 30mm (1 3/16") with mosth averaging about 1 1/4". http://incolor.inetnebr.com/bush/images/CombSpacing30.JPG
They will build a much richer variety of sizes ranging from 4.7mm to 6.6 with most of the brood still in about the 5.1mm range, but some surprising sections of 4.7mm. http://incolor.inetnebr.com/bush/images/47mmComb.JPG
Another regression and you'll be getting a lot of 4.6mm with most of the worker brood 5.0mm or smaller. Why not try it? It's more fun to find out isn't it? If you space the frames closer (1 1/4" like Huber's hive) you will get even smaller sizes.