if you keep them fitted behind the standard grilles, your very limited, and can only go for a basic 10cm unit, which will all be very similar in depth

but if your willing to mount them on the outside of the grille, then you can fit up to a 17cm speaker, and the depth is not an issue as you build a spacer ring to suit. i made up a pair of mdf spacer rings, and then trimmed some of the metal on the mounting frame behind the grille to give extra speaker cone clearance