@jackias-revenge just to paraglide in here with a quick suggestion. You may want to adjust the aspect ratio of your map from a square to something more 16:9 or 16:10. That way when you zoom out in the mapview you won't have dead space. Otherwise it'll give a look like watching a 4:3 movie on a wide-screen, with the bars on either side. You could crop in tighter at the top/bottom, or add in some peripheral areas to the map on the sides, or stretch your map wider before tracing out the baseline.
Also before deciding how large you want to go, make sure you have a unit graphic to compare. 15000 px is quite large.
Here's Aela the Huntress at 54px tall...

Here she is upscaled to 125% (the current ceiling for unit view at the high end)

If I enlarge the Skyrim map you posted at the beginning of this thread to 15000px wide, that Aela graphic, even at 125% of 54px, may be too small to register. She'd be about half the size of the black dot used for Jorrvaskr at that scale. A pretty small Circle, even for a suspicious wolf hehe.
Basically you want to scale your map to the units you plan to use, and make sure the display looks alright for them. You can preview this by just opening your map image at 100% scale and pasting in a unit graphic at 100% or 125% scale to see what you end up with. Then adjust the image dimensions of your map to suit the unit graphics.
Just as an example, here's what you'd get with that unit and a map at around 5000x4000px.

So if you up to 15000 you can imagine map is 3 times as large, but units will be the same size. So it really depends on what zoom you want the playscale to happen at. For something like a a castle or a keep, you could go larger with graphics, but if it's a unit that moves or has to show up in the battle screens basically you got about 68px tall by like 100px for units. Just depends what you're after. Something like a horse or a dragon or a trebuchet you can go wider.