I've used this website before for my previous signature, it should help. http://www.resizeyourimage.com/ It does depend though I think if the image resolution is not clear enough then it would still not look right and slightly blurry, but you can always try it out
Is the version you have saved bigger than this one? It's not possible to enlarge an image for more detail, but if you have a better copy on your computer then you can use that.