You can upload the image to imgur and edit the size there. There are some default options under sizes that will add an additional letter to your url. For example:
You can also edit the size to get exactly what you want (rollover the image > edit image > specify the size). However, as far as I can tell, this changes the image linked to by the original url, so you’ll need to upload the image again.
To link it to the larger size, you can use BBCode tags to link to a different url than it displays. If you want to display XXXXX but link to YYYYY: