I would highly recommend a backpack with hydration capability. Capacity should be based upon what you can comfortably carry. Are you in good enough shape to carry more than 20% of your body weight? Unfortunately I don't know metrics offhand, but you're probably looking at around 30 pounds for every 1000 cubic inches of volume. The hydration bladder will cut into the amount of available volume in the pack. Most bladders are 100 ounces, or 3 liters. Depending upon how long you think you'll need to move, a second bladder may be in order.

Many of your essential tools, such as a knife, flashlight, compass, sunglasses, face mask, communications, money, identification, etc., should be carried on your person, not in the pack. Ultimately the pack may be expendable. Backup water in packets, or a flask is also a good idea.

It doesn't need to be a big pack. Your most important items are water, an emergency shelter, a personal first aid kit, a trauma kit, essential personal medicines if any, more clothing layers for cold or wet depending on conditions, and space for food. Space for spare underwear and toiletries is nice, but you may only need (or be able) to use a change of socks.

Needless to say, this applies to your own needs only. If you are responsible for others, you may need to carry more, but never more than you are able to. Also this only applies to being on the move. You should have a separate kit for sheltering in place; do not cannibalize your evacuation bag for these purposes.