EPSXE depends largely on the game.
If it's a 2d game, I believe the defauld resolution is 320x240, but in a 3d game, your better off putting the res higher.
Also, because the default resolution of RMXP is 640x480, I suggest putting the resolution of epsxe to that, and apply a filter like HQ2x, as it will look better than if you resize it later in photoshop.