Cold is the common failure point
Infrared cabins heat people partly through radiant panels, but a very cold room still increases warm-up time and heat loss. Check the manual for minimum ambient temperature. Insulating or conditioning the garage may matter more than adding a larger sauna.
Treat the electrical location seriously
Garage receptacles may have GFCI protection, while some sauna manuals specify a non-GFCI circuit. Do not defeat protection or improvise an extension cord. Have an electrician reconcile the product instructions with current local requirements.
Separate the sauna from garage hazards
Keep it away from vehicle impact, gasoline, solvents, dust-producing tools, overhead-door tracks, and standing water. Preserve roof clearance, ventilation openings, door swing, and access to the plug or service panel.
Sources and documentation
Specifications, claims, and installation requirements can change. Confirm the current manufacturer manual for the exact model and variant.