Unless you want to have your wife sailing alongside you, having a Rio M and a Rio S
is really not necessary.
Just be sure to get your wife her own ultra lightweight rig (I suggest the Sailworks Retro Ripper as it's one of the lightest and has very good light wind power in a very small package. Pricey, yes, but worth every penny as it will be so easy to use.)
Get her the 4.2 m2 (or a 5.0 m2 if you can find one).
Also, trying to teach your wife may not be the best idea.
It often works, but just as often it can cause problems.
Better for both of you to get some lessons in the basics, to get you started, then decide
how many boards and rigs you will need to buy.
As with everything, there are good schools and there are less good schools.
If you sign up for a lesson, and it's not easy, the conditions are not optimum (wind in the 8-12 knot range is the best) and you are not having fun, find another school.
I've taught hundreds of newbies in the "A Taste of Windsurfing" program and I would say the student success rate is > than 95%.
Wish we lived nearer, as I would love to teach both you and your wife on the 2 Rio M boards that I have.
If you can make a trip to Cape Hatteras sometime, let me know.
Or, I know someone who teaches the same way I do that's in Rhode Island.
Hope this helps,