According to Hoover's website a prairie bluebell egger hen should weigh about 4 pounds, pretty small. They list a brahma hen at 14 pounds. Quite a bit of difference.
Aart is exactly right, as part of the mating act the hen squats, which gets her body on the ground. The rooster's weight goes through the hen's body into the ground instead of through her legs. In engineering terms the pounds per square inch of pressure going through her body is a small fraction of the pressure that would go through her legs. So a hen can hold up a much bigger rooster than you would think. A hen's instinct is to get flat on the ground during mating, whether forced or consensual. That's natures way to protect them. The more difference in weight the more potential for a problem but that potential is pretty low.
I don't know what your goals are as to why you want a rooster. I think you want one that can mate the Brahmas, I did not get any other reason out of you post. I don't know what rooster's you have available from your source. I could assume Hoover Hatchery but that would be an assumption. If you also want to breed those small hens you need a rooster that can also hit the target with them.
The way I'd approach it would be to look for a rooster that otherwise fits your goals and weighs in the range of 8 pounds. That's more for body length than weight. He should be able to hit both targets without hurting either hen.
