The room 13x13. What size rafter should I use?

2X8 #2 hemfir or equivalent according to span tables. Why are you using 5/8 for a dropped ceiling? It's not a fire division. Noise control? It's a killer to hang overhead!

Part of the plaster latt has been removed for mold. So part of it is fire division. Thanks for the help.

Thanks for the clarification. If it is a fire division, make sure you use X rated drywall and they may make you seal openings with fire retardant foam or caulk.