There's this program that I've been using lately called GIMP. It's free and it's sort of like PhotoShop except no extra special tools and filters.
If that doesn't help, use paint...
1 - Open your sinature in paint
2 - Go to the Image Menu
3 - Choose Stretch/Skew
4 - Under Stretch, Enter 77 in both percent fields
This brings the length to just under 500
If you want the height the to be under 100
5 - Repeat steps 2-4, enter in 72 in both percent fields this time
This brings the height to exactly 100, but shortens the signature significatly.
Hope this helped.
Also, I would really much worry about it seeing as no one has really said anything to me about mine being a little to large.
PS - I actached your signature with the edits if you don't want to do all that work.
Edit - Ok, I changed my signature just to be fair.