To add another riders point of view…
I’ve been pretty much only riding 29 for a year now. Have done most of my riding on 125’s and 110’s, with some riding on 150’s and some on 137’s.
Most of my riding has been road riding, I live in a fairly hilly area.
If I could only have one set of cranks it would probably be 137’s. They would be a bit long for road (but not too long, very usable). They would be on the short side for off road use, probably too short, but most of my riding is on pavement.
In the beginning I would have been all for 150’s, it took a while to get comfortable on the uni, but outgrew them after two weeks. Was very happy with the 137 and 125 for a long time. If I lived in a flatter area I probably could have settled into the 125’s for good. But then I started riding longer and wanted to see if I could go faster, went to 110’s. Pretty good for a road machine, I definitely was not as confident with them, mounting became much harder, control in dicey areas suffered as well. I wouldn’t want the 110’s if I was in high traffic areas, or where road conditions were not great.
I hope this helps. The truth is one size doesn’t cut it. 125’s would make the most sense to me in your situation, but there is going to be a longer learning curve at the start. Also every one is different, some people like to spin big, some people like to spin small. Some have good strong knees, some have bad weak knees. It’s good to have a few different sets of cranks anyway.
I’ve got a set of 150’s, 137s, and dual hole 127/110. It really lets the uni shine on almost any terrain, it actually amazes me how versatile my 29er has become.