Originally Posted by Myrdin

Thats why you dont sell gold dragons. Whenever a big dragon drops you keep it.
And you should expand your capacity at leasty by +50.
Unless you plan to get rid of all your 2/3* girls, you will run out of space before long, especially if you start saving up Manyus, Ampys and Dragons for a specific girl you want to pump up from 0 to max in one go.

I can't explain much better why expanding girl capacity is not a waste of FGs. I've seen some people say such, but you simply can't get around it. How much you expand it depends on how much convenience you want and your hoarding habits.

Some people don't hoard dragons in advance, sell all their 2* and 3* girls, and don't mind having to visit the sale screen after every single mission they do, and those people could probably manage with just 25-50 extra slots.
On the other hand, if you do like to hoard dragons, ampies, manyus and keep at least one copy of all girls you ever get, and want to visit the sell screen just once or twice a day, then you'll quickly need about 175 slots. At least, that's the amount I have now, which should be enough for at least another 2 months.