Hi there
I am sure that a shop would measure your finger for you. You could always use the opportunity to try a few rings on to see what style you would like.
I guess you may be from USA. I don't know much about getting married there, but I think you have to both go to your local town or city clerk's office to apply for a licence. It would be wise to call first to check what papers you need to take with you.
Hope that helps
Kate