Thank you for your email. You can view Osprey's specific sizing and fit guide on their site here - http://www.ospreypacks.com/en/product/backpacks/aether_70_1_1
This pack does not have a built in rain cover, but Osprey does sell a rain cover separately that will work for this pack.
Unfortunately, we don't currently have that option on our site, but if you give our customer service team a call at 877-883-6276 we can manually place an order for you.
Please give our Customer Service Team a call at 877-883-6276. They can assist with a belt and shoulder strap swap.
Capacity (70L/4200 cu in) will remain the same from Small to XL. Sizing will be a reflection of torso length. You can view the sizing chart by following the link below the product name on its page. Current promotions are based on the Anniversary Sale that will run until 6/14. No catch.
We are carrying the most current model.
You should definitely go with the medium pack and we can change out the hipbelt for a large. Just call or email with your order number and we'll make the change. In a week or so you will have the option on our site to do that kind of customization.