For small breeding cages suitable for a pair or trio, I put in a roosting bar that is high enough that they do not bump their comb every time they walk under it. Then the space above the roosting bar is high enough that they do not rub their comb on the ceiling. That makes the cage height 1.5 to 2 times the height of the standing bird. Three feet minimum for standard size birds and two feet minimum height for bantams.
Even better (in my opinion) is to give them enough cage height that they have to really jump/fly to get up to the roosting bar. It is about the only exercise they get in a little cage. But, construction costs are always an important factor too.