In London there aren't that many pre-loved designer shops selling enough H bags to try the size of different B & K (unlike in Japan,Paris, or NYC)
At my Hermes store the SA got out a 30 for me just to try the size (he is convinced anything bigger will be too heavy for me) and it didn't make my heart sing, I felt it looked very boring on me even though it looks beautiful on other women.
I did get to try a pre-loved Birkin 35 in a resale store and weirdly I didn't love that one either which I think is because it was black and Togo and somehow it didn't feel 'special'.
I realised that for me the leather is all important and I didn't want a black 35B in a grained leather-it just doesn't make my heart sing.
In the same store I tried a 40 in gold Epsom and actually loved the size but didn't like the colour or the leather-but knowing I have past history with buying bags that become too Heavy (I'm a walker, I rarely drive anywhere in town) I knew I had to knock the idea of a 40 in any leather other than Epsom on the head because I'd be miserable carrying that weight.
a member of this forum
@880 reminded me of the 32 HAC which back in the early 90's was my object of desire.
No longer made I was lucky enough to get to spot one available in a resale store in London (Ardennes leather in Havanne) I tried it and it was like falling in love!
The proportions just felt right on me and I loved the sheen of the leather-For various reasons (mainly boring pandemic induced job anxiety

) I didn't buy it and it sold...
But it convinced me that the HAC 32 makes my heart sing
OR a Birkin 35 in a smooth leather is the way to go.
For me personally the leather is key-the wrong leather would be a deal breaker.