My smallest one currently is a Canson Art Book 180 which measures 9x14 cm (that's 3.5 x 5.5 inches). I keep it near the couch for drawing and colouring in what I call 'tiny mandalas', which is pretty much what it sounds like.
Here are some of the latest I've done:
I draw them with a waterproof drawing pen like a pigma micron or faber castell pittpen and I colour them in with felt tip pens (usually Stabilo 68). They are a nice way to get some quick art done while relaxing on the couch.
I'm looking forward to the day when the entire book will be filled with these. I have a couple of dozen in it now, so still have a while to go.
Wishing you all a wonderful and artsy weekend!