what size belt do i need

Those are the belts you'll need for the ASP underdrive, for the main and the A/C. The OD alternator pulley is smaller than stock, so you'll either need the same belt, or a size smaller. You can get those at Autozone and they work good, I've been running 'em on my car for quite some time. You can probably cross-reference them with gatorbacks if you're looking for those.

