Has anyone used this for a male dog? Do I have to take off this suit to pee?

No, you don't need. There's a zipper on the belly for peeing.

What size to get for a 20 pound dog?

We do not recommend using weight to choose size. The best way is to choose the right size based on your dog's back length and chest measurement.

How to measure dog's body size?

Wrap your soft tape measure around the very base of the neck or chest and leaving two fingers of room. Back measure： Stretch the tape measure in a straight line from the base of the tail to the small indent between your dog’s shoulder blades