Is this unit really as large as the indicated dimensions or are they providing packing box size?

The actual unit is smaller than the dimensions listed under item size. Actual size is approximately 4.5 in long by 3 in. wide (including the buttons) by 1 inch high.

My printer has one port for a usb printer cable. I want to connect two pc,s to one printer. How do I do that?

This is what I use to do that. This hub has 2 printer cable ports, you will connect one cable to each printer. I had to order 1 longer printer cable and I ordered 1 extender. Both owrk just fine. The only caveat after you have both computers connected is you have to switch from A to B. I share the printer with my assistant and it works just fine, whoever needs to print makes the switch. In this picture you can see to round buttons on the front, those are the A and B switch. All in all it was very easy to do!

Is Windows Vista compatible?

It shouldn't matter which operating system your using. This is a USB device so as long as your computers have USB connections and your printer has USB connector, it should work.

Can I connect two printers to one computer with this switch?

I do not have this product (yet, I'm about to order it), but I'm pretty sure that will not work. This has one USB-A port coming in, and two USB-B ports going out. What you need is two USB-A ports coming in, and one USB-B port going out. Fortunately, that type of device exists. It's called a "USB Hub" and they are usually cheaper and more widely available that this. (A hub usually has four inputs, and will allow you to have both printers active at the same time)