This is a list of the screws that are commonly used and the size / specs for them. I always try and have  a variety on hand for mods or repairs.

DIA LENGTH HEAD LOCATION HEX
M4 16 BUTTON X-AXIS BELT 2.5
M3 5 SOCKET HEAD FRAME 2.5
M3 12 SOCKET HEAD 30MM NOZZLE FAN 2.5
M3 5 SOCKET HEAD 30MM NOZZLE FAN HOLDER 2.5
M3 40 SOCKET HEAD 40MM COLD END FAN 2.5
M4 9.5 BUTTON X-AXIS BEARING CARRIER 2.5
M3 3 SET SCREW X-AXIS RODS 1.5
M3 3 SET SCREW Y-AXIS RODS 1.5
M3 10 SOCKET HEAD Z-AXIS COUPLER 2.5