There's no question that backpacks have taken over the travel game. Even if you're not a "backpacker" per say, you'll probably still want a travel backpack with you next time you go out of town, since they're convenient, stylish, and very versatile. Of course, depending on your needs though, the size of the backpack that's best for you may vary. Here's a breakdown of backpack sizes, as well as the different travel situations each one is best suited for.
Small backpacks are best used as carry-on luggage or as additional storage for quick trips to the beach or around town (yes, men, this is just about one of the best ways you can carry a man-purse and not be called out on it). Drawstring bags are among the best for this purpose, as they are almost always made of a light vinyl. They can easily store things like your cellphone, a book or magazine, a snack, glasses, and other small, essential items, making them extremely popular among young people who don't want to look like they're lugging around a library with them. Plus they come in all sorts of funky looks and designs (like the awesome Nuttella bag below). The typical storage space here is about 30 litres.
Medium-sized backpacks look like what you'd typically expect a student to carry. These school bags usually big enough to fit a few text books, as well as a small laptop. In the context of travel, this means they can also fit a few outfits and even a small jacket, making them a great choice as carry-on for international flights, since you'll want to have a few outfits with you just in case your luggage gets lost. They're also very useful for extra storage space and for keeping your valuables close by. Plus, since their target market is students, it's not hard to find a stylish option in this category (like the awesome owl-print backpack pictured here). Typical storage size here is 40 litres.
And now for the big kahunnas. These backpacks are often big enough to carry all of your luggage with you, especially if you roll up your clothes and pack light like a real backpacker. Outdoor backpacks are typically used for these purposes, as these are often the ones you will see young backpackers sporting on their gap years in Thailand. Their design is ergonomic enough to ensure that they beat out other forms of luggage in terms of comfort (yes, including rollers, unless you're only planning on travelling through flat, tiled land), and while they may not be the most stylish of the bunch, they definitely make a statement (which is: I'm a serious traveller). These travel backpacks start at 40L, and only go up from there.