Four reel is the usual size. The tanks are pretty standard, but if you go used, you might want to be sure they use a standard daylight cap. I have an older Kindermann (very high quality, but so are Nikor and most of the others--the tank is pretty low tech) that is just slightly larger than the standard tank size and it doesn't have a daylight cap, so it is necessary to turn the lights off to fill and empty it. The advantage is that I can fill it and empty it more quickly than I could with a daylight cap, but it would be nice to have the option of doing it with the lights on.
For reels, I highly recommend buying new Hewes reels, which are very well made and load easily. They make both 120 and 120/220 reels that fit the standard tank. They cost a little more, but will save many headaches.