For a ceiling height of 2.4m. The minimum room size is a legal requirement. SA, Victoria and WA have further restrictions on the use of this heater type — check with a gas plumber or retailer.
Apart from room size, you also need to consider the climate you live in. For example, a particular 25 MJ/h model might heat 57m2 in a cold climate but 87m2 in a mild climate. Our results table shows the maximum recommended room size in a cool climate.