I bought the small size but it is much too small for my purse. It'd probably work great for a very small, narrow purse. I included pictures with a soda can for size reference.

I just received this bag, and I'm so impressed I had to leave a review. It really is a nice looking bag, and the size is perfect. Just enough for all your everyday essentials, and maybe even some more. The quality of the bag seems good, the zippers all look well made and work great. The hidden zipper side pockets will be great to throw those items in you want to quickly access, like phone and keys. I didn't think I'd like the fluffy little attached keychain, but it's actually cute, not too much fluff and just adds a little extra style. It doesn't bother me, but as some other reviews mentioned it may be made of real fur so take note if that's a concern. Also, as others mentioned it was a nice added touch that it showed up with a scented sachet and smelled good, not that funky smell like some new bags can sometimes have.
The only con I can see is that with the straps in the back, it does add a little difficulty in accessing the main compartment. But to me, that's worth it for the added security of the main pocket being against your back, and the ability to use as a backpack. Also, the side 'water bottle' type compartments don't fit any of my aluminum bottles. But those are just small inconveniences, it doesn't make me love this bag any less.
I can't speak to the durability of this as I just received it, but it does appear to be well made and I don't have any concerns about it. I'll be using it on an upcoming vacation and will be great for switching between using as a backpack and then a purse when you don't want to look so touristy with a backpack. If it holds up well I'll probably order again in another color I love it so much.
Added 6/30: I just wanted to add that after a 10 day vacation to the Smoky Mountains I still LOVE this bag! It held up great, even used it on a few small hikes and was perfect for carrying just what I needed to keep with me. I loved the fact that the main compartment is against my back so didn’t have to worry about theft in the tourist areas. Also, I wanted to add that this bag is definitely waterproof. It rained on us just about everyday and nothing in my bag got wet at all!! Even in a few heavy downpours!