@KevinsWither The 12" square habitat is a great size for an adult. Being mesh that solves ventilation issues, but also causes humidity issues as the species should be kept at 50-80 RH% - so more misting will be needed (and a humidifier especially during winter is highly recommended).
Deroplatys desiccata need at minimum 11" tall and 7" wide/long (27cm tall by 18cm wide/long), and the care sheet linked recommends a habitat that is 12" long x 8" wide x 12" tall (30cm long x 20cm wide x 30cm tall).