If your device's camera doesn't have an inbuilt watermark then no problem, use Samsung "Photo Editor" alternatively for now & there you can personalize the water mark size and position as you want. The Samsung "Photo Editor" activates when you edit any kind of photos from Samsung Gallery.
So open a photo from Samsung Gallery which you've taken earlier then tap on "Edit" button below it looks like a "Pencil" then tap on "Stickers" icon and find out your desired marks to attach on the photo and then save. Screenshots below...
