What Hai Etlik says is true. A round sphere put onto a rectangular flat shape will always have some distortion. You should figure out which type of projection you want to use as there are many options or just go without. This will be relevant for distances because a horizontal line of the same length at the top of your length will cover more distance than if near the equator for example. How your world is spaced out will have political and economic implications if you want to be realistic. Otherwise don't worry about it too much. For example I did not consider projection for my Skenth map initially and got too far along to implement it later.
Your map sounds quite ambitious in size! All that really matters is if your computer can handle working on such a large image smoothly. If you want to print your map such a large map will be useful however as even at 300 dpi such a map size handily fills up many poster sized prints and everything smaller.
For example, a common poster size is 18 x 24 inches and this is 5400 by 7200 pixels if you use the very common resolution of 300 dpi. You may not need to go over that or if the map doesn't need to be super crisp you could set it to 200 dpi to require even fewer pixels to fill such a print area physically. You should ask yourself how large of an image you really need and what you want to do with the image.
It sounds kind of moot since gimp won't even run such a large file but if you can get a hold of another software program like photoshop it may be able to hold larger images.
I would recommend testing your computer's ability to run such a large image first. Create an image of the desired size and fill it with photos and whatever images, exactly what does not matter as you just want to cover the entire canvass as you don't want it to be just one colour as that is easy to compute. Then flatten everything into one layer, copy and paste it a couple of times (maybe 10 to 20 times) and then start playing around with things. Try rotating layers, painting things, adding text, selecting and such. What will probably happen when you are working on your map is you'll have a bunch of layers with a lot of pixel data on them and you'll want to emulate this.
If you find the computer slows down after a while you may (especially in terms of loading and saving by the way) want to consider a smaller size. Increasing the size of an image an image has a non-linear demand on system resources by the way. Increasing a map from 18 x 18 inches to 20 x 20 inches is a lot more taxing than increasing it to 19 x 19 inches. The same of course goes for pixels.
Aside from reducing the size of your image there are a few things you can do to keep the file size and demand on your ram down. One thing you can do is keep your layers to a minimum keeping only the ones you really need and not preserving hundreds of unnecessary layers. Learning what you can merge down and what you may want to keep separate is a bit of an art but it comes with experience. The second thing you can do is reduce the number of undos your software keeps track of. Obviously a shorter history will demand less on your RAM. If I am not mistaken, creating an image in the RGB colour mode may be less intensive than CMYK mode but that is conjecture only based on the observation CMYK images tend to be larger than RGB. There are other things I can't think of and other people may know a few things!
If you are really set on being able to create really a large image you may want to venture into vector art which has a number of limitations and works differently that rasterized (pixel ) art.
The stress testing is just to figure out what your computer can handle so if your machine can handle the shock test at the size you desire, it's all happy sailingWhat cultures, histories, and such you want to have are up to you. I would look into historical precedent for things of course. Consider sites like this as a reference point.
Wow this post got a lot longer than I intended!
Cheers!
-Viking
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