Thinner the outer wire the better basically, it'll heat up quicker with less metal. 26g core 32g wrap is pretty standard but it's far from the best. If you want to take a slow and steady learning curve, certainly starting there is a good idea, but it's not a bad idea to buy 1000' of 36g+ some swivels and a drill and just practice at the point that you'll really love it. You know, slow and steady vs sink or swim method, both are good.
As far as internal diameter, I generally go 3mm though sometimes 2.5, 3.5 or even 4mm a few times.
If you use steam engine to estimate resistance and such, be sure to add the wrap into the calculation, 32g is about .2mm, so a 3mm wrapping rod will be more accurately calculated if you use a 3mm + 2X wrap diameter value, in this case with 32g it'd be 3mm+.4=3.4mm Internal diameter.