I have the Cassie 19, and even though it does have the gold chain strap that could be used to dress it up, I wouldn't consider it a "formal" evening bag - I think it's too big. To me, it's more "dressy casual" or "cocktail casual" if you use the chain. Something smaller like a wallet on chain or clutch would be better for a formal evening event. But of course that's only my personal opinion!
I can't tell you how it compares in size to the regular Cassie or the Tabby26, but I can give you a quick rundown of what I've carried in it:
A smaller wallet (bigger than a card holder, but not as big as a full-size wallet)
Full-size Rayban sunglasses case
Car Key fob with other keys attached
Listerine Pocket Spray
Mini Hand Sanitizer
Compact Mirror
Lip Balm
Pack of Tissues
Face Mask
Phone in the back outside pocket
Even with all that, I still have some room for a couple of small things. I haven't had any issues moving things around to get what I needed in and out. I think it's a really great size!