Print size depends on a number of things. 6X7 can stand the enlargement to make big prints, but 35mm often can not. Where and how the print is to be displayed is obviously important. There are places for murals and other, quite different places for 5X7s. Small picture books can often be very effective.
I usually print 8X10s and 11X14, with 11X14 being my most frequent size for display. I sometimes make 5X7s. Lately I have made some 16X20 prints for display, and I like them. But 16X20 is about the biggest thing I can handle physically, so it becomes a limit.