Hey, welcome to the site!
The battle animations format really depends on the engine you're using for your fangame.
If you're using RPG Maker 2003, these are the sizes you should use for the files:
Image size: 480x480
Each cel in a battle animation is 96x96 pixels. This allows 5 cels per line and 5 cels per column.
Battle Animations (large)
Image size: 640x640
Each cel in a large battle animation is 128x128 pixels. This allows 5 cels per line/column.
RPG Maker 2003 allows you to add more than one cel into a single frame of animation as long as the images are in the same file. You can also make the pictures zoom in and out, or change their shading (to some extent).
I recommend looking up some tutorials online if this info doesn't help.