The PSD header stores width and height in four-byte fields and the format clamps at 30,000 px on a side and 2 GB per file. Documents beyond that are written as PSB, the Large Document Format, which uses the same 8BPS signature with a different version field and wider dimension records.
Readers that claim PSD support usually claim PSB too, but older builds can read one and fail on the other. If a large file opens slowly, it is the composite being decoded, not the layers.
Where this came from
Notes like this one come out of the file library rather than out of a release schedule. The pages linked above are the entries that were being worked on when the note was written, and those entries are rebuilt with each release of the toolset, so a note can go out of date in a way a format page does not.
When that happens the note stays where it is and gets a correction at the bottom rather than being rewritten, because the point of most of these notes is the order things were tried in. The editors who write them are the ones who maintain the catalogue.