How to control 'Ad' size of "First Post Content"

[Brittany]

I am attempting to add a mod to the site that requires the use of the 'First Post Content' ad. The issue is that this takes up so much space of the thread and I cannot figure out where I would be able to edit the 'default' sizing of ads. I have made the content in the space as small as possible but it won't make a difference because of the default ad size.

(I apologize if this is a stupid question but I was unable to find an answer in my search here)