What length is good for swimming?

Distance isn't what you should be aiming for. Although I think a 16m pool is a little short to get a good workout for my taste.

If I were you I would start off with a nice warm up swim. Nothing fast. A couple times back and forth stretching your arms out and trying to get the most pull you can without exerting too much energy. Just something to stretch the muscles out and warm them up.

Then I would do some sprints down and back, think about getting as much pull from your arms and kick from your feet as you can. I know you don't care so much to go fast but this will really help build some good endurance muscles and sprints will get your heart pumping.

Then do something a little bit slower off of that but for a bit longer distance.

You know your body and if you are working out or not. Honestly, do what feels right for you. In reality it doesn't matter so much how far you go but how much you put in to it.

and make sure you allow yourself about 5 mins at the end of each session to do a cool down swim. Like the warm up swim you will be going nice and easy. concentrate on your breathing and pull. This is important so your muscles don't cramp up later or the next time you get in to the pool.

that's one small pool. I couldn't give you a specific amount since I only use 25 yard pools and yours is around 17.5 yards. All I can say is swim as much as you can in that time starting your day off slower and gradually getting faster until you are at a good solid pace. Then slow back down for a couple more lengths right before you finish