Check with Quilts of Valor at www.qovf.org. It will tell you what the minimum size is (I believe it's 55"x65" now), and when you sign up as a quilt top maker, they will then connect you with a longarm quilter. You send the top and backing to the longarmer and work out the rest of the details between the two of you. You (or the longarmer) will need to make a label and a pillowcase to put the quilt in when it is shipped - but all of that info is on the Web site.
So far, all of the quilt makers I've worked with have just wanted me to do the quilting. They make the label and pillowcase and then get information from QOVF as to where to send it to ensure it reaches a service person.
Hope that helps a little bit.