Sorry, I have no idea what you like to accomplish.
Please give a precise description…
Do you want to change the page size of selected pages?
If yes, this could be a two-step process:
 Select all pages you like to change in the Pages panel.
 Switch to the Page Tool and change the size of the selected pages with either the input fields in the Transform panel or the Control panel.
( ACP )
Sorry I was clear! I would like to delete all the custom page sizes. As you can see from the photo, there are so many 'B6' options repeating.
Sorry. I still have no idea where exactly you see this list.
Could you describe in more detail plus a new screenshot that shows more of your screen where you see all the page sizes?
( ACP )
Normally you should be able to choose Custom.. and then edit existing custom presets, but there can’t be multiple custom pages with the same name, so your Pages panel looks like it has been corrupted. UI corruptions can usually be fixed by deleting your InDesign Caches folder — Users ▸ username ▸ Library ▸ Caches ▸ Adobe InDesign ▸ Version X.0. if that doesn‘t work try resetting your preferences
This works!! Thank you!! 🙂