Try out the runway number markings which I show at the end of this tutorial and compare them to the custom ones that you are making. These runway number markings that the sim generates also look a bit aliased for me (which is what I think you’re talking about).
If this is similar to what you’re seeing with the custom markings then it’s more of a sim limitation and just increasing the texture size won’t help you here. You can try increasing your graphics settings but they will only look so good due to how they are rendered.
I’ve heard of people getting around this limitation by importing custom models (just a flat plane) with their custom textures on, I don’t know the performance implications of having all your ground textures as 3D models so be wary of that.