If you're flying Sportsman, get the biggest packs you can fit and stay within the weight requirement (which is padded a little for Sportsman and other classes). The most important thing when you're starting is flight time. Why limit yourself?

I got 4500mah packs for my new 2M airplane - I thought the plane would be heavy and would need the lighter packs to make weight. It turns out I can afford more weight, and when I practice I am constantly pushing my packs to their capacity limits. It's so annoying to be limited to short flights when you don't have to be.

For reference, I'm flying Intermediate. At contests I use anywhere from 2800-3300mah per flight depending on the wind. A 4500mah pack gives you 3600 usable mah if you plan on following the 80% rule. I think my timer is set to 7.5min during practice. Every time I think I can push it to 8min I exceed the 80% rule. 80% rule means you want to use no more than 80% of the battery's capacity, to maintain adequate voltage in the cells (especially under load) and not damage the packs. I miss the 9min flights I had with my 110 size airplane and don't really like flying the last few maneuvers during practice knowing I should be watching throttle on verticals. The best setup leaves you concentrating on flying and nothing else.

A lighter plane will handle a little differently, but any decent 2M airplane should fly just fine at or under 11.x lbs.

I am thinking about getting 5400 or 5800 packs for next season so I can fly a little longer and get a little more practice in per flight.