I'm just guessing from memory
I believe that about four inches they should be ready. When you notice increased red on their throats, a pair hanging out together with one,the male being more defenseive of territory and the female its a sign a bond has formed. Next they will stake out a territory for the spawn and clear the area. They'll clean a flat (rock usually) surface where the female will deposite her eggs. and so on..
The nice thing about firemouths is their less territoral for a central american species, take up less territory and don't leave their territory much to defend beyond its boarders. They just defend when necessary and don't try to get revenge.
Also they're one of the easiest central americans to breed. Good luck!