You can know how big a cell is by it's name, for example a 18650 cell is 18mm in diameter and 65mm in length. If you've heard of 20700 cells then you can apply this rule to know it is 20mm in diameter and 70mm in length.
For 18650, it's an 18 mm diameter x 65 mm length. 21700 is 21 mm x 70 mm.
Some mods can take all 3 sizes of battery- you would get an adapter for the smaller sized 18650 battery
Unless you're already using a Samsung 20S, then yes you can use a more powerful battery.
The mod is variable wattage(adjustable up to 80 watts). Which means it has a power converter, which means a more powerful battery won't hurt it. Might shorten the vape time between charges though.
Ideally you'd want the battery that provides the most run time for the wattage you use/ how you vape.
What specific battery are you using now... and what wattage and coil?