Welcome, Quebec 11.
I'm new to freshwater as well and I've been getting geared up. Check out the great ssticky threads on the Beginner Spearfishing board. They have a good overview of gear.
As far as the length of the gun is concerned, the visibility of the water dictates the appropriate length. A spear will shoot about 2.5 times the length of the spear beyond the tip of the spear. So a longer gun can shoot fish from farther away. The long guns are slower to maneuver and aim, and they are harder to load.
I'm targeting carp in WV, USA. I wound up buying two guns. One lake near me has low vis (6-8 ft max) so I went with a Mako Speargun 55cm. I had the choice of one 16mm band or two 14mm bands. I chose two 14mm because they are MUCH easier to load than the 16mm, and I can choose to load only one of the bands (you just let the second one dangle) if I'm targeting smaller panfish, such as crappies.
The other lake near me, Summersville Lake, usually has excellent visibility 20-40ft in some places. So I bought a Rob Allen 110cm gun. The gun is large.. the 110cm is only the barrel measure and doesn't include the grip or the spear overhang. If I had to buy again, I might go with a slightly smaller gun, like a 90 or 100. If I put the gun on my chest, I can't quit reach the bands. There is a special technique to allow loading it, but a slightly shorter gun would have made it easier for me.
So your visibility will dictate length. You need to be able to see the tip of your spear

plus a couple feet extra.
I'd suggest two 14mm bands. Since you're just starting out, I'd recommend a Mako Speargun. It's very good value. You'll get a very solid gun for the money, and it comes fully rigged with a loading pad. (I had to pay $18 extra to have a loading pad on my Rob Allen gun.). They really stand behind their products, and are a pleasure to do business with.
Also, if you have varying water visibility, like me, you might start out with one target vis and one gun to see if you like the sport.
I hope some of that helps. That's my debrief from my last two months of research and gear buying.