This helps. What I am thinking, however, is 100% realistic image of the town (think Google satellite image) that represents the terrain the player will navigate, with collision boxes of some sort to prevent walking into buildings etc. Would a single image be too big for that to work? If so, does it make sense to use software to cut that image into tiles and simply sew it back together in GMS? Because that terrain would not be uniform I don't anticipate there being too many common tiles to reuse. I am not sure if I have explained my thoughts correctly. Maybe I am jumping the gun, too.
3 things.
1. Getting a realistic game is a very ambitious project and no company has ever created such a game, none up to this date. And yes, some have come close but because of the complexity of the human skin details and how light interacts with the environement, getting a realistic game, for now is impossible without distributing it for extremeely high end computers able to to run tons of image data in memory and this is not a big player pool, we're not all rich. So realism in a game will go as far as what you are capable of presenting to the player and for now, the closest one can get is: "Wow, this game has some very stunning graphics and a heck of a lot of details on everything, from water to skin textures, they thought of everything visually." But you won't have anything that can compare a real life image, really. You CAN re-create a 3D rendered image to resemble another rral life picture, but integrate THAT in an actual game, never seen it because there's too much movement and things going on.
2. If all you want is to have a map and collide with it. I think that 1 single 1920x1080 image won't fill the entire memory of the computer and all you have to do is put invisible objects on top of it. That is as simple as your game can get and NO, the big map size will not be too much for even modest laptops on the market.
3. 3D is where you will get a hit on your performance. YES, you can have a huge map displayed in it's full size, but you need to simulate walls, lamp posts, benches, fire hydrant, grass, trees, etc... These objects need sprites of their own OR tiles of their own because you NEED to draw them in a 3D space OVER your map at different angles depening on where your camera is. NOT only this, if you REALLY want a TRUE 3D effect, you also need images representing the SIDES of your buildings and if you are looking for realistic, you need to have details, broken bricks, scratches, holes, rotten wood, etc... AND to top it all off, to acquire realism, you will need depths on those images (called textures). Depth to simulate cracks and holes, brick bumps, woud and tree height maps AND the light for reflection and refraction. You will also find a lot of work with water effects in case of lakes and oceans or rivers because they reflect city objects with different curves and distortion. So all of these require extra images ON TOP of your FHD map and of course, if you also plan to get closer with your camera, you will need HD wall and building top graphics as well SO yes, you may end up with memory issue if you don't learn to draw things properly in your 3D world which is where optimization takes up your time and where game developpers choose what to draw and what not to draw and up until how far should they draw. You will also learn that LOD details are seperate images and you will need to calculate the distance that will make you draw these low quality images by yourself because the engine won't do it for you.
Normal people would say as
@Rob said, find tutorials and try something less ambitious to first of all get used to the system.
I'll say, we never know when a prodigy can pop up or when a surprise can arrive. If you feel capable of spending the time for such a big game with soooo many details, go for it, but realise that you are alone OR maybe you have a few friends that can work as a team with you. You need a lot of time and patience. So I hope you have that in large quantities and that you also have orders ready on Amazon for extra Patience boxes and Time pills
But I would certainly be curious to see how such a project would develop itself, some people can be very surprising