The bigger tank is better for the fish, but for now it is good enough as long as you change the water promptly.
PUYU is a tough fish, it can stay without water for quit sometime, normally during drought season PUYU and Haruan is the last to survive if they could not sink into deep hole to keep alive...until the next rain come again.
This fish could not grow so big, the biggest i've ever seen is about 6-7 inch.

It got sharp tiny teeth.
You can try feed him with pellet or small fish or frozen bloodworm. It can also feed on insect (like grasshopper) or earthworm.

BTW where is your location..local to Malaysia or elsewhere?

Good luck.