I'll try to tackle this but it could get confusing. Theres a couple ways to do it:
1. You can physically resize the image. Any Windows equiped computer will be able to do this with out any additional software. I use Paint. You can find it through this path: Start Menu, Program, Accessories, Paint. Inside paint, You can paste the image and alter it accordingly. Under the image menu i like to use "Resize/Skew". You change the horizontal and vertical to a size you would like to see. Here is your current sig at 50%:

You can then crop the sides and top and bottom of the pic. Here it is with the wasted space chopped off:

Its important to save the file as a jpeg (.jpg) when modifying through paint brush. Other file types may make it difficult to be viewed over the internet.
2. You can allow the site to change the size of the image with the image code. Just as you would normally post a picture with: [ img ]
whatever the image code is [ /img ]
You can specify the width and height by:
[ img width=800 height=532 ]
whatever the image code is [ /img ]
When doing it this way, you have to be careful not to distort the picture because it could look pretty funny when done. example:
Width 700 Height 200

Width 400 Height 800

Width 700 Height 500

Hope that helps. If you want to see the exact image codes, just click the quote button and you can see it. In order for [ bracket codes ] to display, i have to put spaces in between. The real codes don't use those spaces. Hope that helps.