ORD drainage - changing size of bottom drainage structure (FDOT)

Hello, 

I am working on creating an existing drainage model. 

i drew an existing manhole and connected some 48" pipes to it. But the pipes are now way to big for that manhole structure. I want to make the bottom structure of the manhole bigger. 

I tried going to "open utility properties" and making it a box structure and changing the size of the box to 15' (just to test). But it does not seem to update. 

How can I make the structure bigger to fit the pipes? 

I am using Open Roads designer 10.10 with FDOT connect 10.10 add in.