The general rule of thumb for coop space is 4 sq. feet per standard size chicken. Since Silkies are smaller than a standard bird you could go with a smaller size - say 3 sq. feet per bird.
However, I am a big advocate for going larger than smaller if at all possible. If they are going to have outside run (or free ranging) every single day you can also go a little smaller BUT ... if they may ever be shut up (like due to bad weather or something) for a day or more then go larger for their sakes. Having chickens in too small or cramped space leads to feather pecking.
Silkies need their roosts lower than other breeds but you still want to have them higher than the nest boxes. Thus, I would suggest you put their nest boxes either on the ground or just a couple inches off the ground and then put their roosts just above that.
My silkies are still young so others here with 'real' experience will hopefully chime in and give you better or additional advice.