Whats the right battery for the job?

[SouthOzSV27]

Can anyone advise me on whats the best mAh rating for a 2S Lipo being used as a receiver pack. I will be using a UBEC to regulate to 5v.
I want it to be as light as possible but still allow me to get a few runs out the pack.

weight vs mAh

Would something like this work?

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=7306&Product_Name=Rhino_750mAh_2S_7.4v_20C_Lipoly_Pack