I did read carefully. You wrote: “a giant which was about five foot tall”. You need to write more carefully.
I wrote:
“thigh bone of a giant which was about five foot tall.”
Subject of linking verb “was” is “thigh bone”. To answer the question, “what was five feet tall” leads you to “thigh bone”, not “giant”. Giant is object of proposition of. You must have flunked ninth grade English.