Sizes are usually given for before the heat shrink tubing is shrunk. When not indicated, heat shrink tubing shrinks down two times smaller. Therefore in the case of 1/2" tubing, you could expect to snug up any bundle of material between 1/4" and 1/2" in diameter. However, it is best to not trust in vague descriptions and have the target diameter fall well within range rather than use a given size tubing at the minimum or maximum.
Braided cable sleeving should work like so: compressing the sleeving should let you accommodate a higher diameter, so you will want to size it accordingly, or slightly undershoot. To an extent, you can fit a higher diameter cord (or bundle of wires) at the expense of lost sleeve length.