Honestly, I think there isn’t a big difference within reason. e.g., I really don’t think there is a difference to use a 80 mL Gaiwan vs a 120 mL Gaiwan. Now, if you use so much water that it is close to the rim of the small Gaiwan, then it will be a mess. If you use too little water in a huge Gaiwan, then the water may cool down too fast and difficult to pour.
The size thing is a bigger issue for Yixing purple teapot and particular for Oolong tea. For Oolong tea, the argument is to use as small as a teapot as possible and with very little air space to not allow the tea fragrance to escape and as little temperature change. For other tea, the argument is less. In fact, I have heard many people want more air space and more escape for Pu Erh tea.
Gaiwan (porcelain or clayware or even glass), unlike Yixing clay teaport, is not suppose to impair favor to the tea leaves, so you can brew different tea leaves with the same Gaiwan teacup. Some may say you shouldn’t do that for Yixing teapots. Now, I do recommend your husband and you get different Gaiwan. Mostly because Gaiwan teacups are not that expensive and your husband’s hand size may be different than your hand size. You should pick a Gaiwan that feel comfortable to your hand.
You can get Gaiwan with a wide price range. I have seen $5 or less Gaiwan. The white one you see above is $15 US dollar. The second blue one in the middle is $60. Of course, some can go to several hundred dollars. I would recommend you try a $10-20 one at first.