Can you use this as a cross body sling? I'm 5'4 and 150 lbs.

Yes, the stock strap on this is MUCH longer than earlier models and editions. If you are wide at the chest, there is also a 5" extender strap available for about $10, that works on the current re-design of the locking buckly (the cylinder-in-cylinder design). I have the old and latest model. A year or two ago the strap was shorter, and there wasn't an extender (except for the Fastex style snap in plastic buckle design). Now both problems are solved. Also the one I just got in 2020 has a more supple, less stiff, waist strap, they really are perfecting the art of non-slashable, but still flexible, straps. The originals were thick and unwieldly.

Is this big enough to fit a dslr camera (nikon d7500) ? anyone using it for this purpose?

I'm not a camera buff, and don't know how big that particular camera is, but my gut feel is that this bag is too tight for any sizeable camera or one with a substantial lens. It's more geared toward stowing flatter items like your wallet, checkbook, pencil-case, calculator, chargers, notepad, keys, glasses, and even sheets of 8 1/2" x 11" paper folded in half (with some corner bending). But I'd definitely look into finding a larger model of this same brand as I've found it to be well constructed and I really like the zipper locking gizmo. Of the many "survival packs" I've purchased over the years, this is probably the best (and most stylish) one I've had. I purchased it around a year ago and, with daily rigorous use, it still feels like new.

What is the hip measurement?

Not sure of the exact measurement but it is very long. In my opinion it is way too long. It is one size fits all so they made it for the largest person possible. My waist is 34 and when adjusted to fit me it has excess enough to go around nearly a second time.