John uses the Greek word machaira
From Strong's Lexicon for machaira
1) a large knife, used for killing animals and cutting up flesh
2) a small sword, as distinguished from a large sword
The word machaira
itself comes from the noun mache
meaning "a fight or a combat."
The use of the word machaira
occurs 29 times in the bible, and except for the few times when Jesus refers to Himself as a sword, and when The Revelation refers to a "great sword", the word machaira
indicates a heavy dagger or very short sword as normally carried by civilian travelers and merchants for self-defense.
Such a short sword would have measured between 13.5" (also called a golem
or short cubit
) and 18" (a cubit).
If straight, it would have most likely had a leaf shape, similar to what some readers might call a Keltic blade; and, if curved, it would have followed the pattern of the various Arabic self-defense blades, and it would most probably not have exceeded 13.5" in length.
If I had to make a guess, I would go for the 18" double-edged, leaf-shaped short sword carried by civilian travellers and merchants of the era.
With this blade a person could parry, thrust and hack, making it a true sword.