i am currently working on a android game by using libgdx and i am designing graphics with ps for it. I want my game looks the same quality for different screen sizes but i dont know in which size should i design my graphics. For example if i design the graphics 200x200 size and for a bigger screen i should make it bigger by resizing with code. But this will demage the graphic which is something i dont want. What i think is to make a much bigger graphic then shrink for ever screen sizes. is that right approach to do? Or there is a better way. And also if this approach what size should i use for portrait game. Like 800 width is good? Thanks for ur answers.
I'll assume you are talking about a 2D game or at least a 2D UI. Well, if you think you need to create graphics for 1920x1080 then you should go for it. If you setup a viewport properly libgdx does all the scaling for you. Personally I have my viewport represent a 1280 x 720 area. You won't see any difference on phones, I barely even notice the difference on descent sized tablets. But I'm not a great 2D artist, lol. Anyway this saves a ton of work so keep that in mind.
So what you basically do is create graphics that fit your viewport or camera size. If you are setting the screen size to 1920 x 1080 I guess you should always create your 2D art for that size. For 3D it's different, higher resolution will give you crispier edges on your models. Here I would use a high resolution camera for rendering the 3D and a lower resolution viewport to overlay the UI, again unless you want your 2D to look really crisp on bigger screens.