The pistons are the size of a quarter,nickel or a dime and about 8-12mm thick..Some calipers have one,two and four pistons....a tool isnt necessary with these chinese crap bikes...I can remove the pistons with a screwdriver or o-ring pliers and reinstall them with my thumb after I mod them with my Dremel............even installed and full of fluid they arent very hard to compress...If I may:
Hydraulic brakes work like any other hydraulics. The fluid being pushed in enters via a very small hole in comparison to the vessel it is filling. This multiplies the force and gives you enormous clamping power.
In contrast, it also makes them a devil at times to compress your calipers so that new pads will fit.
The pistons should be fully collapsed down into the caliper bores before you stick on new pads. This will make the installation much easier. With a car its often done with a caliper compressor. A simple tool.
With something like these little bikes, a C clamp can probably be made to work. If its a single piston, it'll go easy. If it's a dual then it may want to push the opposite side out as you compress the one. (same circuit, path of least resistance)
Hope this helps.