There's really 2 factors to keep in mind when choosing batteries. Overall capacity, and current draw.
@WRYZ926 mentioned the first, as this will play into how long you can run portable. It's been a bit since I checked the numbers (and don't have the radios handy to check), but i want to say it was around 4a on high power, which would would buy something around 2.5 hours of transmit time on a 10aH battery. realistically you're likely listening a lot less than transmitting, so the real world use will be longer.
Second consideration, generally, is current draw. Less of an issue with these smaller radios, but we have to consider how much draw the battery can handle. the cheaper LiPo batteries i have refer to not drawing more current than their aH rating, which for mine would be a 10a draw on the smaller, and 20a on the larger (mine are Ecoworthy, and the 10aH is roughly the side of a standard red brick).
Here is a quick example, and their table claims 5.2aH, and can handle a 15.6a draw, and if one isn't transmitting much should be able to last a couple hours at minimum.