I think a 6'6" egg is too small to start on for adult beginners. Unless you are young or have lots of time to hang around paddling, bigger is better.
I am 5'2" and 110lb and had trouble catching waves with a 7'6" by 20" by 2 3/4" minimal when I started in my 30's, although it had a bit of rocker for a minimal. I ended up with a 6'10" McCoy nugget (6'10" by 21" by 3 1/4") to learn on.
For me, I found the hard part catching waves, so more volume made it easier. Once I caught the wave, the standing bit was relatively easy. Paddling was hard to learn for me.
After 2 1/2 years of surfing once a week (spread over about 8 years with numerous breaks for babies), I have a 6'4" by 20" by 2 3/4" McCoy nugget. I could go a bit smaller for a bit of more manoeuvrablilty, and possible duck dive, but even that is borderline.
If she can already read the ocean and pick waves then maybe a bit smaller would be OK. I suppose it depends whether you want her to have fun and catch waves and continue to surf, or just get cranky.
I think that you are a brave man to try to teach your wife to surf , particularly if you can surf well and have forgotten how hard the basics are to master.
Good luck with it. Tell her that Husbands really are lovely people, particularly if they read stuff over your shoulder.