A full size scan on my HP printer would be about 2000 x 3000 pixels, which would be 6Mb assuming no compression.

My pics generally go 900 x 1300. If the rest of the scanner bed scans, I cut the relevant bit out and reduce to 90% (in Paint). That gives me roughly 800 x 1200 JPEGs which run about 300Kb and provide ample detail.

And are quick for the prospective buyer to upload. I would back out very quickly from items that had 7Mb pics, due to the time AND the bandwidth gobbling.