I believe the size is M10 x 1 (pitch) and it is 31.35mm long.
For those of us who have sheared off a stuck nipple and couldn't get the remains out I came across this on Ebay UK which includes a 11mm drill, an M12 x 1 thread tap and a M12 bleed nipple. If you already have an M12 x 1 thread tap then the bleed nipple is available on its own.
M12 X 1.0 Repair Kit with Stainless Steel Bleed Nipple Screw for Brake Calipers | eBay
I can also confirm what I read on a different thread that welding a washer over the broken nipple and either building up the centre of the washer with plenty of weld to get something to grip or then welding a nut to the washer worked for me. I think the heat from the welder helps a lot and the washer protects the caliper body.
Threaded Mode
