Their physical size depends on which one you get. Look at the dimensions and check that the one you want will fit.

They are better than the maxamps packs. It's just a matter of whether you want to spend the money. The FPs are great, but loosing one in a crash sure would hurt a lot more than loosing a maxamps pack. The maxamps packs also work very well for the money. I'd just stick to the packs based on either the 5Ah cells or the 4Ah cells in parallel configurations.