I've downloaded Monster Rancher 2 PSX ROM from www.romulation.net. But when I played it on emulator, it's screensize doesn't fit to my monitor, and also when I burned them into a disc and played it using Playstation, it's screensize also doesn't fit to my TV, there is a "black space" on the top. What's wrong with that?
its nothing to do with the rom. Black bars mean that the content is designed for a different aspect ratio to the display you're using (widescreen vs non-widescreen) and it is done to avoid distorting the picture by trying to make it fit the aspect ratio of your display. Playstation uses a fixed resolution (slightly less than 800x600) because it is designed for standard definition televisions, which (non-widescreen) are 720x576 in Europe and 720x480 in the US. While you can change the resolution on an emulator, there will consequently be some quality loss and possible distortion.