My other pair of Chaco's (Z1) are a size 7.
After reading all the reviews, I decided to order the flips in an 8, just to be sure. To my surprise, the 8 was too small!
Most of my shoes are between 7 and 8 depending on the brand. With sandals, and especially flips, I want some footbed sticking out there to protect my big toe, which is, while not excessively large, tends to stick out where it doesn't belong, and it gets stubbed easily. No fun.
I returned the 8 and ordered a 9. They are perfect! Plenty of room, and no stubs yet...

On a one-to-ten scale I'd give this bag a six.
I bought it just before a 3 week Amtrak trip, when my old bag busted a zipper. Backcountry delivery was very timely, as usual.
The bag arrived with the strap sewn on twisted. Because I didn't have time to return it, I took it to the local shoe repair guy, who corrected the error in less than five minutes for $3.00.
The spec's say the bag measures 9"X9"X3". That 3" is actually 1.5" so it is smaller than I had hoped. However it still carried all the stuff I needed.
It was very easy to find and grab things in a hurry. In a drenching downpour, nothing inside it got wet. It repelled dirt, also and still looks good after three plus weeks of abuse.
The clasp is easy to use, even for an old lady with carpal tunnel.
Several times my pens came loose from their little pockets and slid out of the bag, despite my efforts to secure them. A better design might be to put the pen/pencil pockets vertically inside the bag rather than horizontally in the gusset part of the flap.
A wider strap would be nice, too. The strap is plenty long enough, though, for wearing across the shoulder for security.
And most of all, I wish it came with an external pocket on the back. I travel quite a bit by Amtrak, and coach boarding is kind of like the Oklahoma land rush. Coach passengers must have their tickets handy while boarding and carry all their stuff. It was awkward and slowed down the boarding line when I had to open the bag and remove the ticket. It was equally clumsy to carry the ticket in my hand while also carrying a backpack and two duffels.