Size DOES matter (somehow)…
As a plus-size traveler, everything you will carry is a bit bigger. Bigger pants and t-shirts, bigger swimsuit and sweater… you got the point. You might have to use a bigger size backpack than other more “standard size” travelers – I own 70 liters – and shouldn’t feel bad about it. Though I recommend not going above 75-80 liters of capacity, it would also be tough to go below 50-60 liters.
Do you want to have a bag that’s loaded by the top or the side? Both ways are fine, and nowadays, lots of bags have zippers and drawstrings allowing you to have the best of both world. If you know you’ll be facing severe rain/snow/bad weather and/or that you will be carrying a heavy bag (below 50 pounds is pretty standard), avoid zippers and side-loading as it may weaken the zipper(s) and allow rain/snow to get in your pack. (Strategically placed straps can help diminish the pressure on the zippers.) But know that top-loading forces you to take out a lot of things every time you need something. Unless you plan serious hiking in very nasty weather (for which a cover won’t be enough) and/or a super heavy bag, side-loading or side and top-loading will do just fine.
If it doesn’t fit, you must quit!
Properly fitted clothes are sometimes very far from our reality as chubby persons and budget travelers. But there’s one type of fitting that should NEVER be neglected: your pack’s fitting. (Don’t worry, this kind of fitting is way less uncomfortable than all the fitting you had before!)
You will find online many charts online on how to measure yourself and adjust your bag. Though it could give some advice, I strongly recommend you go buy your bag in an actual store. Bring a flexible measuring tape (like the one seamstress uses) in case the store doesn’t have any (which would raise serious questions!). The salesperson will be able to (help you) measure yourself (some measures are taken on your back or can be tricky to do on your own); if you’re not comfortable being measured by a stranger, bring a friend to help out!
It is a 5-7 minutes step that will make all the difference. The Mountain Equipment Coop (MEC) store associate who was there when I bought my first backpack was able to adjust the height and tension of the shoulder straps and a bunch of other fancy details that made the bag seems way lighter and allowed a better weight distribution over my body. Proper measurements will also allow you to find out if you need a “standard”, “short” or “long” version of a given bag, based on the length of your trunk.
Hips don’t lie
Do not neglect the hip/waist belt, no matter what. This is not a 10 lbs day-pack you’ll be carrying around; most of the weight you’ll carry, if your bag is properly adjusted, will stand on your waist and hips and will give the rest of your upper body a break. Not having a hip/waist belt or not using it because it’s too short is just not an option. Luckily, there is less “size segregation” in that field and you can find bags of all prices with belts long enough to accommodate you.
Put on some weight
Now that you found the right backpack, don’t be shy and walk around with it! Go up and down stairs if you can. Don’t forget to try the bag with some weight in it. The store should have what it takes; go with at least 20 lbs but up to 40 if you can. See how it feels and discuss any discomfort with the salesperson and make sure that every question and comment is addressed and taken care of. This is a crucial moment and a serious investment, as a bad-fitted bag will end up in pain, incapacity to carry it and could even cause injuries.