A question just struck me today. When you create a custom-sized map in the editor, how is the tech rate determined and can it be adjusted later?

It goes off whatever size you modified...you can adjust it in the editor.

So if you create a map which was originally "standard" and you make it 200x200, going in later and enabling custom rules for that .bic and changing the tech rate on the standard world size changes that map's tech rate? I was going to ask how you would know the size used if someone else created the map but I guess you could just change all 5...

That what it seems like to me, based on the editor setup for Civ3.