Maximum commit message size

Empirically, I think the answer is no. This worked (that's a ~100MB commit message):

yes | head -c 100000000 | git commit -F - > /dev/null

Command parts explanation:

• yes repeats "y\n" forever
• head -c 100000000 takes only the first 100,000,000 bytes (~100MB)
• git commit -F - commits with the passed-in commit message (this won’t work if you haven’t staged any changes to commit)
• > /dev/null hides the output from the command, which includes Git repeating back the very long commit message

https://github.com/git/git/blob/master/strbuf.h defines the len field to be a size_t. So at the very least, the maximum length has an upper bound at the maximum value of size_t on your platform of choice.