I can’t help locate or provide PS2 ISOs or instructions to download copyrighted game files.

When searching for the "best" version, you aren't just looking for the smallest file; you’re looking for stability. Here is what to prioritize:

The standard Bully PS2 ISO usually takes up about 4.2GB of space. However, through advanced compression techniques like LZMA2 or 7z, "highly compressed" versions can shrink the file size down to anywhere between 1GB and 2GB. This makes it much easier to download on slower connections and fit onto SD cards.

Avoid any file named Bully_PS2_Highly_Compressed_300MB.rar from unknown YouTube links. Those almost always:

You're looking for a highly compressed Bully PS2 ISO file. Here's some information and a download link (though, please be cautious and ensure you're downloading from a reputable source to avoid any potential malware):