The size for the length of the shaft would be the thicknes of the leather you will use, from the gaurd to the underneath of the mushroom with a little room, as for the mushroom head the diameter wide enough too stop the leather slipping off. Best guess would be the diameter of the mushroom head 5-7mm? Now for the brow plate, the Romans had two methods for stopping this first the soldered the brow at the front, the next is the brow had a key put at the back of the brow which then fitted in a slot cut into the dome. Unfortunaly polishing the brow plate will not stop scratching metal on metal
since you may not be able to do the soldering a cheat would be to use say a nail possibly 2 Mark the brow plate where it sits drill one/ two holes through, then but you may need to take off the brow, drill two sockets into the brow., about 5-10mm in. Once the gaurd is back on you can glue the pins ( I would use a flat headed nail for this ) through the dome into the brow you can get a metal epoxy putty which can also be used, this can all be hidden from the inside if you can try and keep the pins as tight as possible into the brow holes
The next if you can and if there is room put another hidden rivet next to the one holding it on, I would again use a flat headed nail for this , drill through both brow and dome counter sink a little into the brow then put nail through cut of excess and peen over. Then file/sand down to hide rivet
hope this is of help, if anyone has another method please post
otherwise good luck Romvivs
Regards Brennivs