What is the largest quilt that will fit into a regular quilting machine?

If you are referring to a standard sewing machine, that can sew through all the layers ( quilting ) then your problem is not the size of the quilt or the machine, but the accessories. There are items you can buy ( like "Jaws" ) that will hold your quilt rolled up so that you can roll it up to do quilting work. There are a lot of cool gadgets and accessories to help you machine quilt.

Talk to the ladies at your local quilt shop . . . I am certain they will be able to help you select something that will work for you.