To the best of my knowledge, even large fowl silkies are still tiny little things. So the space requirement would be less. It really comes down to your design. While mine can easily house 30 big birds comfortably in a 6x8 space, I incorporated the vertical space (8' at one end sloping to 6' high at the back) into my design so everyone has a place they can go. The only time it gets crowded is when no-one wants to go outside and they all want to dust-bathe at the same time! For mine, an ideal number of really big birds is 24 or less.
Being that yours won't fly, with a clever design involving ramps and different levels (somewhat like a rodent cage), that 7x7 is now 7x7 + 3x3 + 4x2 or whatever... see where I'm going with this? As long as there's also enough ventilation to move all their air. The basic rule is, figure out how many you want, then build for at least double! So yeah, I would say 7x7 will do well for 24 little silkies, maybe as many as 30 with some clever interior design
