Sorry for the late reply mate.
What scale you should use will be dependant on how crisp you want it to be at higher resolutions. If you want to have it playing on a tv for example it would need to be pretty big.
Images will slow down your game when you get to the point where you have too many. How many is too many? That is dependent on your computer or device. Setting up a pixel art game with linear will mean you will reach that limit sooner than if you used point sampling.
If you want to know what the limit is for your device then just keep adding in more assets until you notice slowdown. As Ashley would say, test early and test often.