Best size of heat shrink tubing?
I have some typical 22 AWG wire here and 1/16", 3/32" and 1/8" diameter heat shrink tubing that has a 2:1 shrink ratio. In the not shrunk state each fits over the wire. In the shrunk state the 1/16" and 3/32" grip the wire firmly, but, the 1/8" is just slightly larger in diameter and slips. Don't forget that you may want a fatter solder join or two wires to be under the shrink wrap. For that reason my recommendation is to buy all three sizes. The stuff is not that expensive and it is good to have a good selection around. 

You can buy a kit that contains several diameters like this for under $20:



I got mine for $7 when it was on sale.