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