Never use a shifter on those, there's a reason shifters are nicknamed nut ****ers!
Same goes with the open end of a spanner, dont use it!
Best way i find is to crack it with either the ring end of a good quality spanner or a good quality socket. Then put the ring end of the spanner over the nipple followed by the hose you are using to bleed the caliper. Finishing up is the reverse of this.
If you have 6 sided sockets these are less likely to round a dying or not perfect bleed nipple as they tend not to round things off.
Or you could get a proper slotted brake nut tool thingo from most auto shops.
As for size My R32 brakes are 6mm if my memory serves me correctly, and the others were either 8mm or 10mm. They will be metric though.
Just for the love of all that is holy DONT use a shifter or an open end of a spanner on it!!!!!!