The size and height of your sauna will dictate just about every other aspect - the number of people you can fit in, the power or capacity of the heater, and of course the location you choose to install it in. If you're thinking about installing a new sauna, you should already have an idea about this - whether it’s from the available space you intend to use, or the amount of people you hope it'll be able to house. People tend to overestimate the size they need. You should remember that a larger sauna will take much longer to heat, and will require more power to keep hot.
First, consider the location that you’ve got earmarked for the sauna. Most residential saunas are about 1.8m x 1.8m x 2.1m (6ft x 6ft x 7ft in imperial measurements). A sauna this size will comfortably allow for a top and bottom bench, and leave enough clearance for the heater. You may even be satisfied with only one bench or require a slightly longer room to allow you to lie down, but minor variations of less than about half a metre on any given side won’t make a huge impact on the end result. Anything more will probably require a larger than standard heater size.
Number of people
The next thing you need to consider is how many people are going to be using the sauna at once. Will it be only one or two people, or do you intend to make your sauna a social gathering place? This will help you determine the amount of bench space required. You will need around 1.2m clearance from the heater, or people sitting too close will get burned.
Will people be sitting up or lying down? Stretch out space in a sauna is great, but if you’re converting a broom closet under the stairs, it may not be practical. You will also need to allow room for an outward swinging door that's not going to be in the way or potentially be blocked by other doors.
Height has the greatest impact on the end result in a sauna. Because hot air rises, if the ceiling is too high your heater will have to work much harder to sustain a warm temperature at lower levels. On underpowered gas and electric models, this may trip the emergency cut-off switch and you will need to reset it. As a general rule, saunas should have ceilings of no more than about 2.1m (7ft) in height; anything higher is wasted space anyway. The smaller the room, the more efficient your sauna will be.