Welcome to the "movement", Buyobuyo!
Please allow me to take advantage of this opportunity to sermonize for just a bit ...
Bugging out is something that should be undertaken in only the very worst, and most threatening situations. And even then, leaving the safety and familiarity of your home and community should be done only IF you know exactly where you are going ... and hopefully how you will get there.
What I'm getting at is that truly serious preparedness requires multiple levels of thoughtful planning. Bugging out is really way down the list of priorities in a good overall approach to crisis readiness.
First and foremost, prepare your home as well as you can. Work your community so you know people and they know you. Get familiar with the area and its assets and resources. Stock up. And so on.
OK--some folks say that they live in a place that is not defensible or that it is a disaster waiting to happen. Well, for those that feel that way--the number one step in your preparedness journey ought to be getting thine-self into a better place ... today or tomorrow. Not the day after all hell breaks loose.
Assuming now that you are indeed squared away with all that hunkering-down-first planning ...
and if you know that you're bugging out only as a last-ditch move, and you do have a solid plan as to where you can go to regroup ...
and hopefully you have prepped that site with supplies to some extent ...
then your bugout baggage will reflect that plan. It will be what you need it to be to get you to where you know you will head.
So I have to have you ask yourself these questions:
What will that take to get you there? (I don't know what it will take.)
Are you alone? Are you fit? Do you have multiple transportation options other than going on foot? How far do you have to go? What, who, and how many obstacles are between you and your objective? What kind of potential climate challenges need to be planned for? How long might it take you to get to your destination if you do have to go on foot (worst case)? Do you have special medical or dietary considerations? What are your budgetary limitations?
What kind of experience do you have in crisis? Is your mental make-up such that you know you will have no problems dealing with violence, mayhem, and chaos? Or will you require social support of some kind to get you through? How about your spirituality and/or moral foundation--how might that play into your approach to disaster? (For instance--will you help others or fear and hinder them; avoid or seek them out; see them as helpers or as prey?)
Bottom line: Think it through. Personal circumstances and choices go into how one wants to be truly ready to deal with the worst days of your life. The better prepared you are for it, the more apt you are to be able to get through whatever comes your way.
(Finally--a more direct response to your questions--I would go big with a good-quality soft-sided bag or pack. Having extra capacity is far easier to deal with than not having room for what you need to take with you.)