So I made a model for the main character, which I think should look better then the rest of the models in the game, like AI, trees and all that stuff.
So I am guessing 2048 would look best, I still need make the texture but if I UV unwrap the model it looks like a texture with 2048 would look a lot more detailed then a 1024, is that true and what is the difference performance wise, is it gonna really mess up the Frames per second, or is it going to be fine if it’s just for 1 model this way.
This is for desktop by the way and not a mobile device.
Thanks in advance 
There is no “correct” answer to this question. 2048x2048 (potentially) stores more detailed textures. 1024x1024 will result in better performance and less memory footprint.
As @JedBeryll said though, you should always create your assets at the highest resolution, and then it’s trivial to reduce their size in Unity’s texture import settings. You can then compare yourself the impact on performance/quality difference in a few clicks. Unity - Manual: Texture Import Settings
Note that there are many properties that influence the quality of a texture other than just its size. Good quality textures, with attention to lighting, normal maps, additional decals, and avoiding compression artefacts will likely all have a greater effect on the visual quality of your game more than just having bigger textures. This article is 10 years old, but still relevant - I’m sure there are plenty others out there: Making Quality Game Textures
I don’t think it messes up performance too much if there’s just 1 texture with that size. Also texture sizes can be changed with the quality settings so if you set it lower it would just use 1024x1024 or 512x512 and so forth. You should use a large texture and in the texture import settings you can override the size for your target device if it affects performance too much.