I'm tempted to say, "150 mm, and don't ask why,"but that would be
unfair.
Seriously, though, 150 (or 152, there's no real difference) will be
about ideal.
170s or 175s will be a bit slow and ungainly.
140s will be pretty much the same as 150s.
125s will be just a bit tricky.
110s or shorter are too specialised.
So, 150s it is.
They will give you sufficient control for easy freemounting and idling,
sufficient torque for reasonable hill climbing and forays off road, and
a small enough pedalling circle for reasonably smooth riding on the
flat.
Right now, your question is a bit like, "I am about to buy my first
fishing pole; should it be 10 metres or 11 metres?" The important thing
is to get something you can use and enjoy. Later, you will either:
a) Lose interest, in which case you need something easy to sell, or
b) preferably, get really into the sport, and develop your own tastes
and preferences, based on experience and opportunity. (No good buying a
top spec MUni if the only place you can ride is your local park; no good
buying a top spec freestyler if it turns out that you keep being tempted
to ride tricky bits of off road.
Good luck.
--
Mikefule - Roland Hope School of Unicycling
Everyone should be fatuous for 15 minutes.
------------------------------------------------------------------------
Mikefule's Profile: http://www.unicyclist.com/profile/879
View this thread: http://www.unicyclist.com/thread/34024