I was looking this on e__--Bay but I dont know if this is the correct size. Item no: 121053225975.
Sorry, I dont have experience on this trade. I dont even have idea what soldering iron is good for once in awhile use.
The variety of sizes will ensure something fits
I usually go for something about 4-5 mm in diameter. Fits well around the 14 AWG wire I use (don't forget the shrink tube has to fit around the insulation too).