If you want to get a hydration backpack, get an actual hydration backpack that has pockets and such, and not just a hydration bladder pouch with a pair of shoulder straps attached. I have one exactly like this one that I use very nearly all the time. It fits a 3L CamelPak bladder just fine, along with anything you'd need to get by for 24 hours in reasonable weather. IMO, it's the perfect size for a go-bag (not bugout bag), and it's grey-man enough that you don't draw extra attention like you would with tactical gear.

Short distance hiking, running, festivals, amusement parks, bicycle and motorcycle riding, are a few things I've used mine for over the last 3 years, and it's nearly in as good a shape when I first bought it. It doesn't feel as durable as the cordura material in most tactical gear, but when you're not worried about dragging bodies + gear through the dirt, you end up saving weight and having something that will last you just as long.

As far as bladder size goes, get the biggest one that will fit in whatever bag you choose. 3L/100oz seems to be about the upper limit for bladder size. I'll echo the other poster about bringing an additional water bottle or two as well. A pair of 1L Nalgene bottles would be a perfect addition. My personal experience says that a 3L bladder and a single 1L bottle is more than enough for most day hiking, especially if you have a fresh water source nearby and a means to filter it. However, having more water doesn't hurt. I would suggest a canteen if your environment has fresh water sources that are few and far between. You can loop the carry strap and attach it to your bag with a couple of carabiners, but can also keep the canteen in your vehicle.