Measure the space, not the sauna
Note the width, depth and ceiling height of the intended location, then subtract clearance for the door swing and any heater guard. Also measure the delivery path: doorways, stairs, turns and gate widths decide whether a crated sauna can physically reach the room.
Read capacity honestly
Capacity figures generally describe seated adults on the benches. If you want to stretch out, look at the length of the longest bench rather than the headcount.
Leave room around the cabin
Manufacturers publish required clearances for ventilation and service access. Plan for those before you commit to a location, and confirm the exact figures on the product's specification.
When you're between two sizes
Consider how often more than two people will use it, and whether the larger cabin changes the electrical requirement. Sizing up sometimes moves a model from a household outlet to a dedicated circuit.
This guide is general information, not installation advice. Always follow the manufacturer's instructions and local building and electrical codes, and use licensed trades where required.