As a serious big tote lover I have always been inclined to aim for the HAC or B40 but these days it is really about function and purpose. No birkin yet, but have had a chance to try on 35, 30 and 25.
I want an everyday, casual and light weight (mini) tote bag. Already have travel bags and work bags so 35 was out.
I had initially been concerned about a 30 being to small but after trying it in my opinion it is still too big for what I carry on the daily. I also had the chance to get my HG small Bottega Veneta Cabat that lands a size between a 30 and 35 but is significantly lighter as it is lamb skin. I know it is a very different bag to a Birkin but I don’t want or need a repeat size.
After a lot of considerations, for me, it is now definite a 25 that I want. (I had such a great experience with the nicest SA ever at my store that really gave me time to try on everything and browse before adding my wish).
The only con I can find about the B25 is the handles, but honestly I really don’t mind it as much as people complain about it. The 30 definitely had super comfortable handles but the 25 is absolutely OK, maybe I have small hands but personally I am used to my briefcase being hand carry only and the benefit is again that the bag will never get too heavy because there is a limit to what you can put in it.
To echo the advise of others on here I would really think about how you will use the bag.
Here is a mod shot 30 vs 25.
For reference, I am 173cm or 5,8”, athletic build.