High resolution contact pictures in Marshmallow - what is their size and where are they stored

Assume you're using an AOSP ROM, you just missed the correct answer:

/data/data/com.android.providers.contacts/files/photos

Whatever the original size of the image is, it will be resized to 720*720 and shrinked to less than 64 KiB.

To verify this by yourself, get these commands executed in a root shell. You need BusyBox or Termux for find and xargs Android M introduced ToyBox which has find and xargs built in.

# cd /data/data/com.android.providers.contacts

# ## Add a picture of your choice to a contact

# find -type f | xargs ls -t

See the output. find -type f outputs every single file found under current working directory, and xargs use piped stdin as arguments to ls -t, which lists and sorts all files from the command line by last modification time. You will see that the top 3 files are

./databases/contacts2.db

./databases/contacts2.db-journal

./files/photo/3 (num. varies dpd. on your env.)

The answer is clear as seen above.