^ What Feline said.
Of course you also need to take account of the shape of the neck, not just the thickness - it all makes a big difference to the feel.
As examples of typical necks, a new Fender Strat or Tele has a "Modern C" shaped neck and is about 20mm at the first fret, 22mm at the 12th.
A PRS "Wide Thin" or Gibson "60s/Slim Taper" neck is pretty similar to those Fenders.
PRS "Wide Fat" or Gibson "50s/Rounded" are chunkier.
Most Ibanez necks are much thinner!