I thought 20" was 20" for whatever wheel size
Has more to do with seat to pedal distance, pedal length & standover height.
Road bikes are a bit different, the 20" probably don't apply.
Check "standover" clearance on a mtb. If you have to "Flintstone brake" Good to have an inch or 2
FYI, I got a 29er, I don't do the highly technical single track tails,
The 29er is more comfortable ride, bigger tires go over bumps better & have over 1000 mile riding paved roads with it.
If buying from a bike shop, they should fit you
If not, have a bike shop fit you, then buy wherever.
If a bike shop treats you well, you'll give them your business & go there for parts & service