Hm... Well, you can get Dolphin to output each frame as a PNG file when using frame dumping by setting DumpFramesAsImages = True under [Settings] in GFX.ini, but I'm afraid there's no support for JPEG.
As for the sizes, if you disable Graphics > Advanced > Dump at Internal Resolution, the resolution of the saved images will be determined by the size of the game window. So you can resize the window to a small size to make the saved images small.
As for the sizes, if you disable Graphics > Advanced > Dump at Internal Resolution, the resolution of the saved images will be determined by the size of the game window. So you can resize the window to a small size to make the saved images small.