Hy,
Adsense sizes are fixed. You can choose predefined sizes in your adsense account.
If you want to put bigger ad under your posts you have to put the code of bigger add (728×90) and then style the placeholder with css.
You have to extend the main-content to desired width first through Graphene Options/Display/Columns width options.
Example:
You want to put ad with 728x90px. Than you have to increase the main content to at least 748px (728px add + 20px left&right margin) and if you have 400px sidebar width than you have to put full content to 1148px (748px+400px).