PS2 games can use CD or DVD media. The input size depends on the game and on whether the release expects a CD image, single-layer DVD, or dual-layer DVD. That number alone answers most of the online-patcher question. A browser tab is the wrong place to hold a dual-layer ISO plus a delta. I still document the format because people arrive with a small xdelta that targets a file, or with an old PPF that the engine can apply. PPF is the PlayStation patch format from the disc-swap years. PPF3 can record a source size. It is not the format current groups prefer, but the module applies it.
xdelta3 is the modern path. The same secondary-compression rule as everywhere else applies. Desktop xdelta3 with streaming is the tool I would open for a full image. Delta Patcher is the graphical front end on that path and has its own guide. The browser remains fine for a delta that is a few megabytes against a file you already extracted. I am not going to publish a maximum that the xdelta authors did not publish.
If you have a readme, follow its input ISO description: region, serial, and whether the image is scrubbed. Two ISOs of the same game can differ by padding.
Related pages: xdelta, Delta Patcher, GameCube if your image is the smaller disc, and the formats index for PPF's row. This site does not host ISOs.
