Hi! Congratulations on your new bag! I am the same height and a size 4 US. I have the 36 in Blue Agate (i think mine is a little more of a muted blue) and I don't think it is too big. But you have to consider what you use it for. I mainly use mine for travel and when I need to carry a bunch of stuff. There were times when I used it eventhough I didnt carry much because the color went well with the outfit. LOL! And with jeans or shorts, the 36 looks great. I have the 30 but I usually use it with dresses for some reason.
As for the slouchiness, I have the Vache Country leather so I am not sure how fast the Negonda leather will sag or if it will ever sag. I don't see sagging on mine yet but I do expect it to happen over time if I always carry heavy items like my laptop in it. I try to only carry light items or not over fill it. Perhaps consider an insert or an acrylic liner for the base of the bag?
Thank you

I've only ever seen Garden Party with Negonda leather recently but it's good to know that it comes with different leathers too! Yes, absolutely, all of my bags are fitted with custom made bag organizers so I think if I look after it properly it shouldn't experience any sagging at all.