I have the tiniest Shorai battery they make in my XS650. Kick only, MikesXS regulator/rectifier. Works like a charm. The only difference between batteries is the ampere-hours and the cold cranking amps.
Ampere hours is basically the capacity of the battery - how many amps it can deliver continuously for 1 hour (without being charged)
The cold cranking amps is how many amps it can deliver at the freezing point temperature wise. It's how much oomph it can deliver to an electric starter.
So, for kick only you don't really need CCA. The battery just has to be strong enough to spark the plugs when you kick the engine over and the ignition fires. The rotor/stator in an XS650 are not powerful enough to do this on their own - it needs the battery.
A Permanent Magnet Alternator (PMA) is more powerful in terms of how it can create more amperage at a slower rotation so it generates enough juice to fire the plugs when you kick it. An SR500 has a PMA and a battery, but you can replace the battery with a capacitor to complete the charging system (you can't just disconnect the battery - electrons need a place to go).
For the money, buy the biggest AHr battery you can in the case size you want from Shorai. They make different capacity batteries in the same size cases. You don't save much by buying the smaller one.
When they make a recommendation, they're basing it on equivalent AHr and CCA to the stock battery, hence the larger size. But you don't need that unless you're running the e-kicker.