It depends on what you're designing the game for - consoles, mobile or PC.
If PC, then your canvas should be at least 1920 x 1080 and any backgrounds you create should be around that size too. The game elements will be whatever size is needed for them to fit the background correctly.
Keep in mind that devices can scale down better than they can scale up. But don't go to big, because then the game's performance will suffer.
If it looks good in your graphics app it will look good in Unity, as long as Unity doesn't have to scale the asset a lot.