I'm 5'3", 110lbs, and have noodle arms if that helps 
My Neverfull MM is awesome with casual and since it has minimal Vachetta, it's great in all weather conditions too! 
If you find it too big, go for the PM - it's SOOOOOO cute!
If you find it too big, go for the PM - it's SOOOOOO cute!
You lucky girl! The neverfull in MM or PM would be great for you, as would the Monty GM. I cannot get it to fit comfortably over my shoulder with a suit, I am heavier than you, but a little taller. I have a mono Musette, but it would be too big on you, since your petite and it's long, not wide. It holds slightly less than a Speedy 30, anyway and is pricey at a thousand bucks, so I would really go either the Neverfull or the Monty. (they have smaller Musettes with a long strap, you didn't say size and they dress up or down and are available in Damier or mono, but I am leaning towards a Monty MM or Neverfull for you.
