国立美術館 : マップと概略案内. (OCLC #906972890)

RDA 3.5.1.4.11 on Dimensions of Sheets says, for folded sheets not intended to be read in pages, “record the height × width when extended followed by the height × width when folded” as in:

46 x 46 cm folded to 12 x 23 cm