To what depth is this watch water resistant?

It meets the standard ISO definition of "water resistant" which means it can withstand being at a depth of 10 cm of water for one hour. This is why the watch is labeled "water resist" without a depth mentioned. A diving watch it is not and I would probably avoid intensive swimming with it. Showering and hand washing are fine. I've used mine casually in a pool a couple times without issue but I wasn't diving under the water.

What is the length of the watchband?

The length of the watch with strap is 9 inches, or 23.5 centimeters. It is a simple buckle design, just like a belt buckle. My wrist is 7.5 inches, and it fits comfortably. The designers thoughtfully put the longest possible row of holes in the strap, so it should fit on a very small wrist (it should be easy to snip off the extra length of the strap with scissors). The strap is rubber. It is very flexible and comfortable. Hope this is helpful.