CNC Height?

Hi everyone, I want to decide on what height I want. I can’t decide between 124mm (5") or 74mm (3"). I think 74mm (3") would be too short, but 124mm ( 5") might make the machine too nonrigid (I’m planning on a lot of aluminum and hope for some steel). The machine size itself will be 500x500mm (20x20"). Which should I build?

Thanks everyone!

4" is the recommended size, so go for that.