That is interesting; that problem did occur with a previous version of the OCRhelper (see here for example). I have not come across it recently; none of the files I have just checked after recently OCRing them using DT are showing incorrect page sizes. They all show ABBYY FineReader Engine 12 as the creator. (As an aside: all my scans are ISO sizes, e.g. A4, A3.)
Could you pls post the version number of this file: /Users/yourusername/Library/Application Support/DEVONthink 3/Abbyy/DTOCRHelper.app on your Mac?
Mine is 1.1.17, dated 09.02.2021.