ThumbnailLength

(called JPEGInterchangeFormatLength in the specification, this is ThumbnailLength in IFD1 of JPEG and some TIFF-based images, IFD0 of MRW images and AVI and MOV videos, and the SubIFD in IFD1 of SRW images; PreviewImageLength in MakerNotes and IFD0 of ARW and SR2 images; JpgFromRawLength in SubIFD of NEF images, and IFD2 of PEF images; and OtherImageLength in everything else)

Also known as: ThumbnailLength, ThumbnailLength, PreviewImageLength, PreviewImageLength, JpgFromRawLength.

# read
exiftool -ThumbnailLength photo.jpg
# write
exiftool -ThumbnailLength="…" photo.jpg
# remove
exiftool -ThumbnailLength= photo.jpg
# all files in a folder, as CSV
exiftool -csv -ThumbnailLength -r ./photos > thumbnail-length.csv

In Python with Pillow:

from PIL import Image
exif = Image.open("photo.jpg").getexif()
value = exif.get(0x0202)  # ThumbnailLength
print(value)