Read this there is a section about reducing file size in this tutorail. You can also use Vexorian's Optimizer.
EDIT: Optimizing your map will alos protect it and make it impossible to open in World Editor, so make backup.
I would take out any added music or sounds(take up a ton of room), and also compress as many files as possible. I believe there are a few file compressors on this site somewhere, just do some digging. If worst comes to worst take out anything unnecessary such as extra terrain.
Optimize the map with vexorian's optimizer. That removes all editor only data (like the GUI data). Using JASS can still yield gains in filze size due to more efficient program flow and structure but those are nowhere near as big as from GUI to optimized map.
The map limit is as follows and is given in common unit meaning (not actual unit meaning).
Maplimit = 8 MegaBytes = 8192 KiloBytes = 8388608 Bytes
It is impossible to exceede this limit if the map is intended for online play.
Inorder to reduce filesize you will need to delete data from your map (remove content). Imported art is usually the largest user of space so start there. Optimizations to object number and triggers can also reduce filesize but very little.
If you use WEU, it is likly that it has bloated your map with over 1 MB of crap which you will need to remove.
Compressing graphics with a higher or more efficient compression can cut file size substantially as well as redcing the pixel size. Eg icons with pallet compression or loading screens with a larger JPEG compression setting. Reducing an image by half in both demensions will quarter its file size.
Yes having a 1080p loading screen is nice but you really can not afford the filesplace and most people will not notice the difference with lower resolutions.