The alignment of the cylinders, the HPs, where in the powerband the engine makes it’s power, the ergos, frame geometry and bike weight are all huge factors that determine whether or not a given bike is suitable for a novice rider.
An argument could be made that an 883cc Harley Sportster (maybe even a 1200cc Sporty) is a safer bike to start on than a 600cc Sport bike. I’m not recommending either as a good choice, but the point is that the ccs don’t tell the whole story.
All things being equal, v-twin engines (like in the Stars) typically provide more manageable power in the lower rev range than do parallel twins and much more so than inline 3s and 4-cyllinders. Cruisers also tend to have low seat height that appeals to many novices and frame geometry that is a bit more forgiving of braking mistakes.
Once you’re completed the MSF, the 650 Star may be a decent choice for you. But, it may not be a good choice at all for your wife and the biggest downside is going to be the weight.
A 650 set up like a Silverado weighs in at more than 580 lbs. That’s a beast for a beginner and can be very intimidating. Compare that to the 250, with a curb weight of 324 lbs.
If I were you, I’d reserve buying anything until you’ve both completed the MSF and sat on a few bikes.
Also, you may want to try to put the 2-up riding idea out of your mind for a while. Carrying a passenger creates many issues with handling (not relate to the bike size). It may be more practical to get an older 250 that you can both learn on, then in a year or two, you can upgrade to a 650-ish bike that is capable of carrying two.