Sorry if I posted this in the incorrect area, newer to the forum.
So I want to create UI’s for my game. Such as a health bar UI, stamina bar UI, Menu UI, Magic UI and items of the sort such as those. Are there any specific dimensions they need to be? For example the standard PC background size 1920 x 1080. Couldn’t find anybody who had asked this question before, that’s why I asked.
Thank you very much! 
To put it simply: all of them. Roblox has a mix of people playing in fullscreen mode and windowed mode, and window sizes can vary a lot. Some people will use split screen so they can have a passive game on and a browser or Studio to do something else. Ideally, you should design your user interface to work on all resolutions and aspect ratios for both landscape and portrait. Additionally, make sure your UI scales, because some players can play at displays as low resolution as 480p on mobile to as high as 4320p (8K) on Desktop.
Thank you, was very helpful. Appreciate it.