I recently acquired the Lihit Lab Teffa Bag in the A5 size from Jet Pens. It comfortably fits my medium Ecosystem Life Artist notebook (5.25x8.25") with room to toss bits of paper, letters and other items and still successfully zip it closed.

The outside of the bag has two velcro pockets, both gusseted; one comfortably fits a mobile phone, the other holds business cards, stamps and other tidbits. Behind each velcro pocket is a slit pocket to keep more goodies and on the far left is a pen pocket. I pack this thing to bulging and the 2-sided zipper makes it easy to close even cram-packed.

Available for $14 in lime green, orange, black, and brown. There is also a larger size to hold A4 for $18.50.