Create a project with 500x500.
Set to letterbox mode.
Set size to 1000x1000 and snap. Even 250x250 if you want.
Will get result of width and height depend your browser and resolution.
So if you use the same thing but use off mode and add any size you will get the same size.
I use "canvas size" before "take snapshot".
Simple lines of code:
1) Set canvas size
2) Take snapshot
3) On canvas snapshot -> Invoke download
Is there any different way i miss to get any size with letterbox mode?