The size of the self tapping screw is 6mmx16mm, tapering from 6mm to 6.3mm.
If the c-shaped washer was installed correctly you better find out why it came off. Extrusions too long; touching the terminal / luff of the sail too long; halyard
swivel hard against the c-shaped washer / lose set screws of a bearing holder; extrusions came apart etc. Most likely the bearing holder slid down inside the extrusion, check the bearing for damage.