Cont...
7mm is good for any flat wires (eg.. transformer lead to digital piano) and 13mm is my go-to for most bigger stuff..
I got split tubing cos it's easier to fit over (though can still be a PITA!) and for things like piano transformer wrapped electrician's tape around the very end so it doesn't easily shift and expose wiring - everything else I just had the tube.
And yeah, about $8-10 for 3m, depending on diameter, at Bunnings. All the best with it!