I'd actually say you want 3, one small (1/4) for fiddly, one medium ( 3/8 ) for all round and one BFO (1/2) for beefy.
If you only get one though get something like a 8-60Nm 3/8, covers most of the fasteners you'll need on the bike. I've got one of these:
http://www.halfords.com/webapp/wcs/s...egoryId_165469
Which has seen a *lot* of use.
Then I have the bigger 60-300Nm one which hasn't seen half as much use
http://www.halfords.com/webapp/wcs/s...egoryId_165469
Druid