The size of the size of the wire (cross sectional area not length) depends on what you want to do;
Power the starter motor - then you'll need somethign the same (or larger) then the wires used on the bike ..
Charge the battery - this will depend on the amount of time you have avalible - the more time the less the thickness of wire.
----- For flexable wire with a good variation of size I find "Hi Fi" speaker wire to be Ok... not looking to power the starter motor - just charge up the battery enough that the bike will start.
----- For clips - you want some that fit your bike ... I use a large one for the chassie connection and put that on an engine bolt.. The battery positive terminal is alsways hard to get to .. and needs some care as it is usually sourounded by other stuff.
Regards Frank Warner
motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S