That's too long for Simple Delivery so you have to try and opt out and buy your own postage.   During the listing process after selecting category and entering item title scroll down and click on the box under the heading Package Size.   At the very bottom of the pop-up you should see: Oversized.  
Select that and you should get: Domestic postage/Simple Delivery is unavailable for this item.
Then click on Add Primary Service to buy your own postage.

It has to be done on a laptop or desktop rather than the app.  If the option doesn't show up try changing sub-category or category.