Finding Big Map Size

I’m working to make a mini-map I have the picture of the whole map and a minimap gui. The issue is that it requires the size of the map, I’ve tried making a part covering the whole map but my map is way too big. I also tried unions the parts but that only made it smaller. Any fix?

Your map is over the studs 2048 by 2048. You can subtract distance from 2 points to get the size.