There is an active thread here addressing line sizes and diameters

http://www.rcuniverse.com/forum/m_10777142/tm.htm

with lots of 1/2A info. Short answer is that you don't need .012's for
a 1/2A trainer and unless you are contest flying, AMA doesn't have
any 'approval' for 1/2a's.

Dacron 1/2A lines, .008 braided steel, spiderwire braided fishing line,
there are lots of choices which will suffice until you move up to
something bigger/faster/heavier.

One small warning is not to use too large a handle - you want the
lines spaced fairly close together at the handle to slow down the
controls.