Yes, don't purchase the pack online. Go to a large outdoors store such as REI. Try on every pack they have. They will have sandbags to put in them so you can get an idea of what the pack feels like loaded. Everyone fits differently. Pack frames come in an assortment of different sizes. Beyond that, brand fit is like ski boots. You may fit Salomon ski boots, but not Nordica ski boots. You may fit a Gregory backpack but not an Osprey backpack.
You probably want a 45l or 55l backpack.