I love pixel art for one main reason....You are allowed to ignore details. Now, as basic as this may be, I also learned that details in pixel-art goes further than just a single sprite. I prefer to take pixel-art games as a series of layers super-imposed one on top of the other creating the final big picture. Although, not to everyone's taste, I really love how beautiful Pixelnicks did their Eagle Island game. Many subtle details make the game vivid, living and feel HD even as pixel-art. Basically, you get pixel-art trees in 2 or 3 background layers. Then you have all those mice, moles, frogs, fish, clams, flowers, dandelions, even the swaying hanging post lights and hanging vines. A game is like magic and your goal is to trick the player's eyes so he/she does not concentrate of a single detail.
When your player runs on the screen, you want him/her to concentrate on the whole picture and not on the single character. Creating the whole environement to distract the player's eyes (not game play) will most definatively use lower res graphics while enjoying adding extra details to make your game live. You watch a Magic show and are amazed by the final trick because the Magician does not let you see the lady moving from one place to another and covering up areas you should not see.
GameMaker Studio has some cool tools for that and one I love the most is particles. Particles, in pixel-art games, can take up so much space while giving you illusions of detail. When checking Super Mario Bros 1. They used the same sprite for clouds and bushes and simply changed the color. No one really took time to examine the bush and clouds while playing, we are concentrated on not being killed and gaining power ups. If you look at Super Mario World, Mario's face, it's all a question of details about when to draw white or black, a mustach or a mouth. Even though You draw little mixels, you still need to be clever in when to draw one and when not to give the illusion of something bigger than what is drawn.
If you are really wanting to make a pixel-art game, for me, 256x256 is enough for a huge sprite, ok, go for 320x320, and 32x32 pretty good for smaller ones. And 16x16 for the tiniest details you want to not spend to much time on. To me those are reasonable if you are looking to create a higher def pixel-art game for 2020 standards. Back then 64x64 were considered big sprites but we were playing on 640x480 resolution screens. When you are using 512x512 and higher res images, I think you are now starting to look at HD games that are not pixel-art anymore. Pixel-art has, to me, 2 main niches: Simple Static games à-la Super Mario and Sonic style and More complex and detailed like Eagle Island style. Both are good in their own way. And then, you have the HD games where people bring high def images onboard to give us stunning results like the Trine series which are splendid, detailed and beautiful.
If you want your display resolution to be 1920x1080 and that you ARE creating rooms that size or are looking for crisp clear non-blocky images, I do not think that pixel-art is the right term for your game anymore because pixel-art will work at most on a 928x522, to my opinion. Higher than that, I think we are starting to look at games heading for HD.
Again, That's my opinion, anyone can have their's. I hope it may help a bit.