Surprised nobody replied. You might want to try searching the forum for earlier discussions. The SA spearos are a good source of info on such things (Oz, Florida, Californian , Hawaiians & Polynesians too). Threads with contributions by forum member Miles would be a good place to look. He is "living the dream" as a Bluewater Tuna fisherman with a passion for railguns and first hand experience of most/all. I think they consider 110cm-120cm a good all round gun out there, so bluewater sizes would probably be bigger sizes: 130/140/150cm ...I think somebody even mentioned 170! When sizes needs start going much above 140, I think folk often start looking at big multi-band wooden canons - the American spearos would be the ones to ask about those (e.g. Bill McIntyre), although Miles also uses SA Tommy Botha wooden tuna guns.
You might also try contacting Rob Allen, either through the forums Private Mail - he is a forum member - or via their website:
Rob Allen spearguns, railguns and other spearfishing and freediving accessories.