It is such a personal decision. I remember when I was first asking for help with a Birkin and was debating a 25 or 30 (mind you I am 5'7"). Someone here on tpf mentioned I would be good with even a 35. They were right. I won't even consider a Birkin smaller than a 35 now. I have been fortunate enough to see a 30 in person at the boutique, and I can tell you it looked very small on me with my height (more like a lady like purse). Not that there was anything wrong with that. But to me, that is where Kelly fits the bill. Everyone is different.
I would ask your SA to check into your SO. Let her make some calls and find out if it went through, etc. I have heard of box birkins taking a long time, but 4 years seems particularly long for your SO.