CREATE TABLE table_31385 ("Value" TEXT, "Dimensions" TEXT, "Main Color" TEXT, "Obverse" TEXT, "Reverse" TEXT, "Watermark" TEXT, "Date of issue" TEXT)

What are the dimensions of the coin with western sea barrage and locks at taedong gang on the reverse side?
SELECT "Dimensions" FROM table_31385 WHERE "Reverse" = 'Western sea barrage and locks at Taedong Gang'
SELECT "Dimensions" FROM "table_31385" WHERE "Reverse" = 'Western sea barrage and locks at Taedong Gang'