This was an issue, and very correct to consider, with sauna heaters that have a closed body, stones inside, and the top open exposing the top layer of stones there. This is true in wood-fired heaters and electric heaters. Additionally, if the sauna is poorly insulated, or has actual large gaps etc ventilation at the lower level, there is a lot of cooler air coming in. The hot air from the heater makes a gradient layer from the stone level up. However, this is greatly resolved by having the benches at the correct height, well insulated walls and roof, and controlled ventilation, and additionally if there is mechanical forced ventilation pushing air into the sauna from the ceiling level, like we have, evening out the temperature gradient, that helps. And important thing is the heater: Grid-type pillar heaters etc that are open from the sides will heat the sauna air at a much lower level, and really take away that need to raise the benches above the top of the heater.
A great sauna can be made into many various spaces that may have various constraints, and for example a high inside height of the sauna in many cases is not possible to do, while would be ideal, and the sauna will work fine without having it.
It is important to understand that the sauna heater mostly heats the air of the sauna by air passing through the heater. If the heater outer casing is closed, the hot air escapes at the top of the heater, and the stratification, thermal layer, tends to form at that level. In wood burning saunas if there is some smoke in the air this can be clearly seen. The open grid heaters are typically floor standing models. Due to the open grid heaters typically being tall, the resistors inside are also long and span from near the floor to near the top of the heater. So as there is no closed casing, hot air escapes from the sides as well, and already at very low level. Thus the thermal layering is greatly reduced.
Löyly is hot water vapour evaporated from the hot rocks. The difference between closed and open grid heaters is that in a closed heater the rocks tend to become hotter especially at the top level as all hot air comes up through the heater and past the top layer of stones, where in an open heater the hot air escapes in all directions, and there is less of a hot spot at the top. Water poured on stones then evaporates either from really hot stones or bit milder hot, and this makes a difference in the vapour droplet size, so the vapour is "dry" or "moist". Moist löyly feels softer and more tolerable, where dry löyly feels stingy and more agressive. But this is a matter of opinion and preference. A big effect is on the quantity of stones. If there are not a lot of stones, they won't give consistent löyly as they cool down, but they might be really hot in the beginning. Where as a large quantity of rocks, that all together have a great surface area and so even though the rocks temperature is not so high, the steam production ability will stay more consistent due to the stone mass.
Most of the heating by a sauna heater comes from air passing through it, not from radiant heat. The amount of radiant heat is dependent on the heater model, and it is accounted for in the required safety distances to burning materials around the heater, that give an indication about it. If the required safety distances are big then one can presume the radiant heat to be significant, and consequently the heater requires more space in the sauna. We actually look at it from a safety point of view in our sauna designs. We don't want sauna bathers to have to go too near the sauna heater, so to avoid anyone inadvertantly touching or leaning on the heater etc. And from usability/ergonomics point of view, there should be enough space to move in and out of the sauna, without those risks. When that is in order, the heater is far away enough that there is no radiant heat to notice or worry about.
Sauna dimensions are one of the key details in successful sauna build. There is a lot written and and professed about the ideal sauna size, some are adamant a sauna should not be under a certain size. We claim there is no such thing as an ideal size of a sauna. It is a question of ergonomics, ventilation, insulation, correct heater sizing, and optimal bench dimensions and location. A small sauna for only two persons can have excellent sauna bathing quality, and a large sauna can be very poor in that quality. Actually, in many cases a large sauna is worse than a small one, much worse. A large sauna requires much more planning and expertise to achieve a good steam quality and even temperature, as often with poor execution there are cool corners, places where the sauna steam does not reach or where it goes too directly too quickly, not to go into problems with ventilation. An ideal sauna is dimensioned according to the location, the number of sauna bathers, the available electrical connection, and the budget. The functionality can then be designed to be very good in that sauna size.
The typical bench depth is 60 cm, but 50 cm is fine in small saunas. In large saunas where sauna bathers intend to lay down or raise their feet up, the depth can be 70, even 80 cm. Sauna bench width for each person can be calculated to require c. 55-60 cm of space. There is no minimum width other than that consideration. There should be a minimum of 35 cm of depth on the lower bench for feet, but the more space there is, the more spacious the whole sauna will feel. The upper sauna bench height should be 110-115 cm down from the ceiling. The lower bench should be 40-45 cm down from the upper bench. The stepping bench step height should be less than 35 cm, there can be multiple steps, and the steps should be evenly divided for the height of the lower bench. A wall attached heater should be placed as close to the floor as allowed.