I started small, a 29gal biocube, and we agreed if I could keep things alive I could upgrade. The trick was not telling her what I wanted to upgrade to lol! I had a 135 in hand, and in place actually, at one point but the timing just wasn't good. Shortly after got a good deal on a 60 cube. When we moved to Florida, she knew I was going to want a bigger tank. So now I have a 220gal waiting for a few more things before I fill it up. Namely plumbing, rock, and sand.
I'm fortunate that my wife not only tolerates but encourages this hobby. She and the kids love the fish and corals. She doesn't do any of the work, but she understands the expense involved and helps budget for it. I don't need to keep anything a secret. She realizes there are worse, more expensive, or both types of hobbies that I could get into. At least this one the whole family can enjoy. I also gave up pretty much all of my other hobbies to focus on this one. So it's my "one thing" that I do and enjoy.
So, is there any part of the hobby that she enjoys? Particular fish, sea horses, or types of coral that you could accommodate in a larger tank? My wife LOVES anthias. I told her we could get a large harem of them in a 220. Or two harem's of a different species even. Honestly, that sold her. Find something she likes, and then fit it into what you want to do.