If you use 'High quality' fullscreen mode, then everything displays in full detail. As in, yes, if you design hi-res sprites scaled down for 720p and then run it in 1080p, you get more detail.
The 'window size' actually makes very little difference. It's named wrong really - it's just the viewport size, and the default letterbox scale mode will stretch that to whatever size screen you have (480p, 720p, 1080p, 2K, 4K, etc) and simply scale the display accordingly, revealing more detail if there is any to show in the artwork, or reducing it if the screen is smaller.