RE: whats a good battery size for rap 30
I don't think the battery size is linked to the size of the heli ... more the type of servos and receiver you use. If you are using digital servos, keep in mind that they consume more battery power.
You don't have to run two batteries in a heli, just install a battery indicator... if the battery power runs low, swap it out. Adding a second battery will add extra weight.
A 1300mAH battery should last you for quite few flights before you have to swap it out.
I use a 1800mAH battery in my heli with a 1100mAH as a backup, but I have digital servos all round.