Below is a FTU mask by Vix, which is sized 600x600. In Corel PSP, it will automatically look at this mask and add it to any size paper, of which you'd have to re-size before adding to the canvas. In PDN, however, it takes a little work.
Open your mask in Paint.Net and then Image > Resize > 800x800 > ok.
To apply your mask, re-size your paper and then Effects > Alpha Mask > Find your mask > OK.
Now the mask is applied to your larger paper.
Go back to your original mask and undo the re-size than save in its' proper size for later usage with tagger sized kits!