I have the small classic flap and I love the size. I prefer it over the medium. It’s smaller looking that the medium, but in terms of capacity, the difference is slight, at least from what I’ve read. You might be able to hold everything plus two more lipsticks in the m/l compared to the small.
However, I think if you tried on the m/l and loved it, I would stick with that one instead of the small. I feel like with the price you pay for Chanel, it’s not something worth settling over.
EDT: you might also find this helpful:
https://www.lollipuff.com/blog/484/...-flap-small-vs-medium-gold-vs-silver-hardware