Well the size of the one you submitted for the last week was fine. But I would suggest not making them larger than the other entries. It would be unfair that way. And those entries that are obviously larger than the suggested size won't be allowed... If that makes sense.
The sizes are to prevent them from being issues should people want to use them in their sigs on SPPf. They are also a guideline and nothing more than that - its not really something I enforce in the WBC