FYI I actually use 750 x 640 (which is the size recommended in the PICO-8 manual) on all my PICO-8 games on itch and it works great. I think the extra width is so that you can see the clickable buttons on the side (although the layout for those is unfortunately broken in Chrome in the latest version)
I've actually noticed some of the submissions have some scaling/rounding issues where not all pixels are displayed as squares...I'm not sure what dimensions those ones are using but they are apparently not good dimensions to use :p