voltage = how much power it gives out, can be used to increase rate of fire, but can also burn your motor out
Ampage (mAh) = how long it can produce the voltage, ie how long the battery lasts, more wattage, longer lasting battery.
As for max size of battery. Well that depends wholly on the size of your battery compartment, be it a PEQ box or a slot in the foregrip/stock. If you have a look at www.component-shop.co.uk then they give you dimensions of various airsoft batteries they make. Measure the size of the one you have, or measure the size of your battery comparment and compare to their measurements. However if you want a really big battery then you need a battery sling (like a normal one, but it has a battery wire in it and holds the battery on your back). I have a sling for my Real sword 97b as its a pain to change batteries in it and a sling allows me to stick a 4300 in it and got mental all day!