I just measured the spare Bushlite candles that came with the Deluxe Carry Bag and they are about 4.25" long and then the cone-shaped, tip part on top is above that yet. So the grand total length when brand new and unused is about 4.75" long.
They are 20 mm in diameter and that's about 13/16". I did try to see if a normal U.S. typical 3/4" by 10" long, taper candle would fit into the Bushlite and it does, even the slightly thicker bottom of the taper candle. I read on a British website, Bushcraft UK,
http://www.bushcraftuk.com/forum/showthread.php?t=12708& an interesting discussion of the UCO versus the Bushlite and over there, they use ordinary 10" long candles and just cut them in half for their Bushlite lanterns. It looks to me like we can do that here, as well.
The big selling point for the UCO candle lantern is that their special, proprietary candles can burn for around 9 hours. It appears that they are able to achieve that by the way the spring lifts the candle up into the cone shaped top, thus forcing the candle to actually burn all of the candle wax, instead of just allowing the candle wax to melt and run down the outside of the candle and also that they have a special formulation of their candle wax that also assists the entire candle to actually burn and not just melt. I do think that is also part of the reason that their candles don't seem to have quite as big of a flame and therefore not quite as bright of a flame as an ordinary candle, like are used in the Bushlite candle lanterns.
So, to answer your question, "what is the maximum length of the candle that the Bushlite can use?", I would say that even though the actual Bushlite replacement candles are 4" long, I think that cutting an American 10" candle in half, 5" each, would probably work, but it's going to really compress the spring in the base maybe more than it's designed to be. But if a guy did cut a 10" candle in half, he might want to "sharpen" the top of the bottom half to help expose the candle wick to help it light the first time. That would probably help out by slightly shortening the 5" and thereby not over squishing the spring.