I wanna say 1 1/4" I gotta check. I am all out of those. I can't locate them in the stores here and have to order them from casita. Plus when they include the acorn, washer, and cap it makes it convenient. I have used stainless on one or two where the acorn just wouldn't grab.
The length of the 3/16" diamter rivet for the roof will vary slightly depending on the year of your trailer. Older models had slightly thinner fiberglass and therefore took a shorter rivet, about 1 inch. You can get a rivet kit and the answer to your question from LittleHouseCustoms.com. The Casita factory can also supply both but I find them slower to respond.
Trailer: 2001 Spirit Deluxe Casita "New Name: Eggspensive"
We did get a kit from Little house and have purchased others to have on hand but we have apparantly used all the inch ones. My camper is 2001. We have a call into little house and they usually call back quickly and ship unbelieveably fast. Thanks guys.