The first thing to check is the size of your custom sprite. The original Bulbasaur sprite in Pokémon Fire Red is 32x32 pixels. If your custom sprite is a different size, it could cause the game to freeze when you try to select it. To fix this, you need to resize your sprite to 32x32 pixels.
Step-2: Check the sprite offset
The next thing to check is the sprite offset. The sprite offset is the number of pixels that the sprite is offset from its original position. In Pokémon Fire Red, the Bulbasaur sprite is offset by 0 pixels in the X direction and 16 pixels in the Y direction. If your custom sprite has a different offset, it could cause the game to freeze when you try to select it. To fix this, you need to change the sprite offset to 0 pixels in the X direction and 16 pixels in the Y direction.
Step-3: Check the palette
The final thing to check is the palette. The palette is the set of colors that are used to render the sprites. In Pokémon Fire Red, the Bulbasaur sprite uses the following palette:
* Color 0: Black
* Color 1: Dark green
* Color 2: Light green
* Color 3: Yellow
If your custom sprite uses a different palette, it could cause the game to freeze when you try to select it. To fix this, you need to change the palette to the one used by the original Bulbasaur sprite.
After checking all of these things, if the game still freezes when you try to select your custom sprite, then there may be a problem with the ROM hack itself. You may need to try a different ROM hack or consult with a ROM hacking expert for further assistance.