In the back of the owners manual you should find a wiring diagram or schematic. That should have the proper fuse size noted. I think the 'text' portion of the owners manual may be more generic while the schematic will likely be more accurate. An opinion.
That said, unless you have things uncommon to a mower, I don't know what would need the extra amperage, except if your charging system is beefed up to handle an electric PTO, which your particular model does not have.
If you put an ammeter in series, such as across the fuse terminals, you can see the current flow and have an idea of whether a 20 or 30 is appropriate.
tom