Longsword and sword and a half are sort of interchangeable since longsword refers to any sword that is long enough to be wielded two handed. Basically all sword and a half swords are longswords but not all longswords are a sword and a half. They have a blade length of anything from about 100-122cm and about 120-150cm including hilt
The RPG longsword is refered to as an Arming Sword and they have blade lengths of up to 74cm (about 91-100cm including hilt).
Now a Zweihänder could be anything up to about 180cm which IS actually as big as some people (it's about 5ft7in). What can I say? Germans like big ass swords.
As for ōdachi, from what I understand they were usually a little smaller than a zweihänder. The only account I can think of of one actually being used in battle was during the Battle of Anegawa where a samurai fighting for the Azai-Asakura forces used one on horseback. That is it. All other sources I have access to depict ōdachi as a purely ceremonial blade.