You can clear them from the file menu
Adobe seems to keep a list of 20 different document sizes you recently edited. Not directly related to the recent documents edited list hast to do with document canvas size and resolution. I cleared and set the recent document list to length to 10 and to 0. The recent new document size list remained 20 it a different list related to document sized not particular documents. I found no way to change the list length from 20 or clear the list other than deleting the file "MRU New Doc Sizes.json" In your Photoshop Preferences folder. That will cause Photoshop the start a new list when you start Photoshop. It will have a default size and perhaps the current clipboard size. Neither ever used.
do you know where I can find this file on MAC
I would be in your user id Photoshop preferences folder.