The minis have been fashionable, I see more full size now (worn 'fashionably' on commuters/shoppers not back-packer style on travellers).
Most brands now have a small or medium backpack available and they are useful, choose one that looks like a 'normal' bag that happens to be a back pack. Don't get anything that screams cute/ looks like a chid's/has busy print and you can carry on wearing it long after the fashion is passed.
I have a vintage Burberry back pack, it's very useful for the gym, I also have a 'large' Chanel (it's actually not that large, just larger than the small). My
Prada I gave away. I think everyone should have a back pack for those days you need/want 2 hands free.