I have an 8' longboard right now but all my friends either shortboard or moved down to shory boarding. I'm a pretty tall guy and use to own a shortboard and just nosedived everytime...it was 5'6". How "big" can a board be to manuever like a shortboard?
weight im 6'4 and love to ride my 5'8 lost rnf, i weigh 150lbs, weight is def more important then how tall you are, if you are just starting to ride short boards get one at least 2 inches taller than you, and a little wider over all. my 6'6 firewire manovers great, you may want to learn how to do manovers before you are worryed about if the board is not manoverable enough. anything with a pointed nose up to 6'10 should be fine.
its not only height of the board its thickness and width as well and sometimes even the class coating plays are part in it. I'm 5'7" and weigh 135lbs and ride a 5'8"x18"x2 1/4" short board with pretty thick glassing i think it's 4-6oz. I learned to surf on a 5'10"x18 3/4x2 1/2" short board with very thin glassing. I suggest you do not do what i did, i learned the hard way. I think you should go for maybe a wide 6'2-6'6, but i think you should ask a professional.