If you don't have a jumbo muffin tin, you could also use large mason jar lids with the same result. (iheartnaptime.net)
I used a small mason jar lid as my template, and got about 10 circles per sheet (sometimes more, sometimes less). (annehogan.net)
I like to do this by taking a large mason jar lid, placing plastic wrap on the inside of it and then pressing the meat mixture into the lid. (greensnchocolate.com)