cutting size can be anything you want, its your cut is it not?
it can be young or old, short & fat or long & slender, you get to decide.
plump, well hydrated cacti will root much quicker than dried out ones, although they all root at some point in time
Generally speaking, planting horizontally typically results in multiple roots & multiple stems, however you have no control where the stems emerge from the log.
By cutting into 12" cuts & rooting vertically however, you will get bigger fatter plants rather than getting 1-2 thinner ones. A vertical planted center cut trich can grow 1-4 fat pups generally speaking.
A long term issue is allowing adequate grow spacing to avoid crowding as the stems mature and branch. At a minimum, in ground trichs should be no less than 24" apart when planted.
For rooting either vertical or horizontal, a twelve inch minimum cut size for any trich cut is highly recommended and the bigger the better from there.
Do some personal testing, vertically grow a 12" rooted center cut from each horizontal log you plant. Grow them both the same.
Nothing tops first hand experience.