Good to worry, most plastic coops are too small for more than few birds.but worried it’s too small.
It depends a bit where you are; in the UK the term 'coop' means just the house they sleep in. In the US it means a lot of space in 3 dimensions, like an incorporated indoor run. I have plastic coops from Green Frog; the large size would accommodate your Brahmas. But most of the plastic coops I see (e.g. Eglus) are too small for large breeds like Brahmas, and are better suited to bantams; they can take 3 Warrens at a push.So when everyone is saying 30 square feet of space is that the coop only where they sleep or does that include the run?
That is coop space only. Each bird requires at LEAST 4 sq ft of coop only space.... they require 10 sq ft of run space a piece. If you pack them into too small a space, then you WILL have problems.