It's not the size of the arrow, it's the power of the bowman. To coin a phrase...LOL
Typical loose is mediterranian style, index finger above arrow nock with middle and ring fingers below. There are a several other ways to loose the arrow, three fingers under arrow nock is common. Exceptions include those using a thumb ring, korean and turkish archers used this style, only the thumb to pull the string.
Generally arrow length is one inch plus draw length. Depends on physical measurements of the archer. The typical adult male archer has a draw length of 26-28 inches. If the draw and arrow length and bow are matched to the archer, the back of the arrowhead would just touch the index finger of the hand holding the bow. Longer arrows can be used and reasonably could be when there were lots of archers shooting lots of arrows, they would have them long enough that all could be used. The archer has an "anchor point" where the hand is touching a particular spot each time, corner of the mouth is typical. Gets a lot more involved but thats the high points. Some of the publicity shots were absymal but the movie and those above were spot on. dvp