A few of the icons look like they might be under a commercial license, as they appear to have possibly been ripped from commercial games. I realize that the iconset was distributed through the internet, however that doesn't excuse you from not getting the required licensing.
However, license management is not my job. Since you clearly have the icons on you, you could easily find a website online that will take the image and resize it for you. You need to increase the width and height both to be 1.5 times it is now. Given the height of the file now, it's probably going to be a very large file in the end, and that'll make it slow for the editor to open it each time you need to pick or change an icon, and increase the amount of memory that is needed to keep the file loaded in-game.