The size should be on the gypsy, almost certainly 8mm. There is a standard specification DIN766 ISO
Jimmy Green list chains for SL, Lewmar and Lofrans and guess they will probably know which is suitable. Alternatively if you have the manual for the winch it will give the specification
I think that Leroy Somer makes the power side of the windlass - maybe just the motor/gears. The capstan/gypsy is probably a stock piece of kit from someone else, and should be marked with the chain size.