I have started using http://5e.d20srd.org/fantasy/world/ to try to make a good D&D world. Problem is it doesn't say how many miles per hex is. Does anyone know the answer?? If you use Earth as a model, each hex would be 250 miles but that doesn't seem correct.
There are some guidelines in the DMG for mapping your world, starting at page 14.
They suggest that for the widest scale (which they call "Mapping a Continent") that you should use about 60 miles per hex. This will make a world a bit smaller than Earth, but still something plenty big for a campaign.
They've also got some other useful guidelines for mapping out terrain, kingdoms and civilizations that might help you.
There is no standard overland hex size used throughout the ecosystem of published materials. It changes from publisher to publisher, author to author, and sometimes even from one version of D&D to the next. For smaller-scale areas used in hexcrawl play, 5-mile, 6-mile, and 10-mile hexes seem to be common, although I've also seen 24-mile and 30-mile hexes used by authors who like each hex to represent a full day's travel.
For a full-world map, such as those generated by the site you linked, a larger hex size is obviously called for, so that you don't need hundreds of thousands of hexes. I haven't counted the dimensions of the maps generated by that site, but the 250-mile hexes you came up with for an Earth-sized world sound about right to me.