Nice looking speargun. Looking at this page, it appears Cressi have some odd size options:
http://www.tsourosmarine.gr/products_en.asp?prodid=256
i.e. 85, 95, 105, 115
Choosing the speargun size is not an exact science. As my brother observed about shooting shotguns: the more you have the more it seems you find you have the wrong one in your hands for the shot you want to take.
If you have poor visibility where you dive (I would expect poor visibility in N. Devon but I might be wrong), you'll do better with a short speargun (e.g. 70/75cm) - plus it'll be easier to carry/transport & load. If you have clear visibility, a longer gun will give you a little more range (not that much more for a 90) and a little more power - but you'll loose maneuverability in the water.
My current thinking is that for where I spear most (Dorset, S. Devon, Cornwall) 75cm is the best choice for most occasions - as visibility usually is not that good, even in Cornwall, despite its reputation for clear waters. Although I'd be almost as happy to use a powerful 70 (e.g. Rob Allen railgun) or longer light/slim speargun 80 (e.g. Omer XXV 80). One of the Hampshire/Sussex spearos swore by his custom sized 68cm speargun
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My second speargun is 90cm but I almost never use it: when the visibility is clear, I often found a 90 not long enough for the long shots but too long for the close shots. Also, you usually can't tell what visibility is like until you have been in the water & sometimes conditions can change as you get out further; I tried taking both spearguns in the water but it was just a hassle, so I tend to stick with the 75.
My personal suggestion would be to first get a decent but inexpensive 75cm to start with (they are useful to have for poor vis. days anyway & easier to load). Then, if you can tell you will need something longer, buy a second speargun of the length that you now know you need.
However, Pav lives in N. Devon and switched from a 75 Cressi Commanche to a 90 Omer XXV (v. slim & light for a 90) - he spears in Cornwall too though. As I said - it's not an exact science...