*Special thanks and dedication to zapdotep, who discovered this trick in the first place*
Forum thread: http://aoe.heavengames.com/cgi-bin/aoecgi/display.cgi?action=ct&f=4,5829,0,365
This larger-size bitmap also lets you edit the area down and right of the original mapdefault. It also includes a fixed version of the original-sized one, which apparently has been a pixel up and left of where it should have been for the past ten years without anyone noticing.
This larger bitmap requires 1024x768 screen resolution, the largest possible, or some will be cut off. Most AoE users probably use this anyway, but I recommend making sure they use it in a readme or the file description, or better yet both.
The black boxes in the larger bitmap are where the buttons (History, Instructions, Hints, OK) are. They need to be like that because the former three aren't always there and the last one glitches for some reason if you put it there. (Don't worry if you didn't understand that.)
UPDATE: It turns out that you save scenario filesize by turning the areas that you don't change to transparent (black), so do so if you wish, though it's not necessary.
Why 5.0? Because what it does, it does well. It allows you to add things beyond the edges of the mapdefault and into the scenery of the instruction screen. Also 5.0 because I didn't think of it first! :) I salute your creativity!
This file is just a lttle bonus for the really dedicated designers out there.