Your scanning software would be the first place to check, most often they will include a function which will increase the size of the piece being scanned.
I don't use Paint, I have Illustrator, Freehand and Photoshop. Any of which would do this quickly. Likely your Paint program will as well, if 400 percent seems larger than you would like, then pick another number, such as 200.
Does the pattern state a finished size? If so, compare that to the apx. size of the finished product you'd like. We'll here to help you come up with the correct percentage to enlarge your pattern if you need a hand. :)