You likely used the numbers from the THX online viewing distance calculator which shows the THX recommended viewing distance which was somewhat based off of theater viewing, not home theater viewing.
The Wikipedia entry on viewing distance is a bit more current with viewing distance shown as a range from about 28 degrees to 40 degrees as THX recommended.
http://en.wikipedia.org/wiki/Optimum_HDTV_viewing_distance
I would not listen to ANYONE who has any opinion on the first date.
That is, you have lived with 40"-60" displays (most likely) in your home for a while. So, the jump to a true cinematic experience within your home is a new thing. Going to that level of immersion takes some time to get used to and the first time is always going to be the one that is most difficult to get used to.
I don't live in the world of SWMBO, as my wife does object from time to time, but at the end of the day, my wife KNOWS that I've done my homework and she knows diddly about A/V other than she likes her shows, and she prefers them in HD (after I properly trained her on that). It really is up to you how you handle your screen size, but it should be a well thought out decision by you with minimal input from your wife if she isn't willing to put a couple of weeks time into viewing and adapting to a large screen.
I have consistently talked my clients into larger screens than they have originally wanted and not one has complained - and almost every single one has actually thanked me. But, really, the difference between 100" and 108" is not nearly that much. It's like a 54" display vs. a 50" display. It just isn't that big of an overall jump in size. Pick a happy medium and be happy if that's what works for you.