Average size is 1929x1080, so I suggest using Roblox studio and then make the frame sizing and then copy the size as the pixel dimensions and use plugins to convert the frames to images and upload your image of the UI.
Hello there! A personal recommendation I’d give you is, start making it on a piece of paper first, once you’ve got a clear idea of what you want, then move to illustrator or photoshop. As Spiral said, use 1920x1080 as the standard size for the UI
Keep in mind Roblox will automatically resize your decals to 1024x1024, so, make sure they’re not drastically wide or tall, if you want 1:1 quality, upload the pictures already in 1024x1024 resolution