It would depend on the taste of your young couple. I usually stay with a straight line simple font it I don't know which they prefer. I usually will put about about a 4" center letter (last name) and 3" her initial first and same size his last.
David and Jill Anderson would be JAD. This is just my personal preference, you could do his first initial first of just the large last initial. Do you have any fonts with your software? There should be something suitable included as generally there is a variety.

I use very this , that belongs to the "butterhexe 4"... you find in the DBC (page 64)
There you find several beautiful monograms.
Good luck!

http://designsbycuties.cuteembroidery.com/4409674_alpha-quot-quot.html

I just did this for my daughter going off to college. You can use almost any design. Dense is good. This is about 3.5 inches. Sometimes I use smaller fonts and put it in the little stripe that's not terry cloth.
Do not use anything sticky for basting because it will pull the fabric threads out when you remove it. You can hoop a woven water soluble stabilizer (wss). Put a clear wss (the kind like plastic wrap) on top of the towel. Pin carefully then do a basting square around the design before the design is stitched.

This shows a set that I did for my cousin's wedding

Sorry I forgot to upload the pics The towel is a 3 1/2 font and the hand towel is a 2 1/2 inch font

Thank you to everyone for your wonderful help. I will try to send pictures when I have completed the project.