step 1: some time while he's out gather together the biggest bunch of stuff you might need to carry around -- i mean all of it: sunglasses, extra sweater (fisherman knit -- not fine cashmere), magazines, portfolio, laptop (don't forget the cables and the disc drive), address book, ipod . . .
step 2: buy a good bottle of wine or whatever he likes to drink
step 3: take it all out to the coffee table -- fill up the birkin with what you can and leave the rest on the side, with the open bottle and glasses.
step 4: when he comes home you say "sweetie, can you come here for a minute?" and then you hand him the glass of wine and explain how it's such a wonderful bag and you love the idea of having a gift from him that you carry with you . . . BUT you'd be able to get more use out of it if it were big enough to fit the laptop etc. -- so you can use it on those days, too. and what would he think about seeing what larger sizes might work? but since it was a present from him, you want to go to the shop together to talk to the SA about size options and then take him to lunch.
step 5: say 'thank you' with appropriate enthusiasm.