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The Fronter room export packages the room and its contents into a ZIP file. The ZIP file format itself has a limit of 4 GB, which effectively is also the limit for a successful Fronter room export.
Please note this limit is for the resulting ZIP file, not the original room.
As the compression is dependent upon for instance the specific files stored within the room and to which extent they can be compressed, it is not possible to give an exact limit for the size of the original room. Typically, text based files compress well, whereas certain graphic file formats are in fact already compressed and will hence not be compressed further very much.