If i have a platformer mainly with 20x20px tile size and characters around 30x30, what happens if i load for example 40x40px tiles and manually scale them to 20x20 so they have better definition? It affects game performance?
The larger images you load the more image memory will be used. This is based on the original size so if you load 40x40px images and scale them to 20x20px it will still use as if it was 40x40px.
Scaling 40x40 image down to 20x20px wont improve quality, as you have less amount of pixels to show the same amount of information.
40x40 px = 1600 pixels
20x20 px = 400 pixels
So as you scale down big images you loose details and thereby quality, if you scale up images you have more pixels available to show information that there are in the original image, and therefore you will start to get more blurred images.
The best you can do if you want the best quality images is to make them the size you need and not scale them, but in most cases you can get away with scaling as long its not to much.