How to choose the right length for a longbow.
Thanks in advance.  

man that is quite a question there it depends on the style of the bow your draw length and your preferrence.

My advice is to talk to the manufacturer.  It depends on several factors.  You draw length first and foremost.  But intended use, shooting style etc all come into play.    Talk to the maker and let them give you their advice.  Not all bows are the the same.

You have recieved some good advice so far but I have another, try as many bows as you can get your hands on . See what suits you and go from there.  

Depends on the bow like they said.I have shot some 62" bows @ my 28" draw that aren't nearly as smooth as my 58" Dryad Orion.

I like 66in straight limbed longbows, but as stated above I have shot some R/D 58 & 62 that were real smooth its a hard question to answer!