Unity 5 editor set play width and height in pixels

Hello. I want to upload my game into internet. Its set in width 900 and height 750 pixel height and the game gui is very bad everything is not in the place it should be how can i see what will happen in unity editor before uploading it? its possible to set in fullscreen i think but how do i set it to width and height in pixels soo i can see what will happen? thanks.

You can pull the “Game” tab off of the editor and resize it like any other window. Then you can try it out with various resolutions and make sure your UI works for them before publishing.