I just wanted a resolution which give better performance as well as good appearance.
GFX?
“GFX” is very vague, GFX is the abbreviation for Graphic Effects / Graphics. You can be properly assisted once you specify what type of graphics you’re referring to.
As for Roblox graphics, no matter the size or resolution they will always be uploaded. Roblox does have a feature that lowers quality if it’s too high (thumbnails excluded pretty sure) so you shouldn’t worry about performance.
I think 1920 x 1080 or 1280 x 720 (16:9 aspect ratio) is fine for most people especially if you don’t have a fast computer to render. You can go higher to 4K if your computer’s GPU/APU can handle. 1:1 aspect ratio is good for thumbnails.
I assume you need help with Blender rendering unless you are talking about something else, just like @oSudden said you need to specify what you need help with.