Q: what size is the bottom compartment
A: The bottom isn't a separate compartment, it just has a zip located at the bottom so you can access your contents from there without having to remove everything from the top.
Q: Would it be suitable for a size 8 5.1 feet tall lady?
A: Yes, as the back straps are adjustable.
Q: Are shoulder straps extra padded?
A: Yes, they are well padded. The thickness of the straps is approx 15mm.