What size should I make the jpegs I am giving to my client (suitable for 4x6, but I want them to come to me for larger prints)

Thanks!

Well, if they want to enlarge them nothing can stop them, but you can tell them that the quality has been optimised for 4x6 and the pics aren't supposed to be enlarged.

Anyways, to answer your question, crop the pics to 4x6" @ 300dpi and then save them as level 10 JPEGs.