The rivers look better; the px size of the brush doesn't really matter if you are going for an artistic, more abstract representation. That said, it looks good when rivers start with faint, thin lines and grow progressively thicker/darker as they approach the coast/join their courses with other tributaries. You can do this through many methods, although the easiest one is with a digital pen and playing around with the pressure.

There are now some issues with the lakes, as most of them seem to have no drainage into the ocean. Unless you have an Aral Sea-like situation, where evapouration/depletion outweights the influx of water, or endorheic flows (where the water leaks underground, often feedin swamplands), lakes and landlocked seas will always have an outgoing river, as all that water needs to go somewhere. The big lake on the central western continent, the tiny one in that same continent where the southern peninsula starts, those four tiny bodies inside the central island, and the seven in the eastern continent, none of them have outflows.