Hi Aaron,
Rob Allen Tuna is a good speargun/railgun and well able to deal with large fish. The combination of spear diameter has been chosen to fit that powerful band combination. But 2x16mm bands might be overkill on a 75cm-80cm barreled speargun - you might consider the RA Sparid model which is similar but has 1x16mm & a thinner (but not not thin) 6.6mm spear, it also has a double notch spear & double rubber muzzle, so could be easily upgraded later to other rubber combinations (e.g. 1x19mm, 2x14mm, 2x16mm) but actually 1x16mm bulk rubber is pretty powerful as it is: the RA fixed muzzle is reckoned to have about 10cm more rubber that typical screw in bands, which have a much bigger hollow in the middle (i.e. RAs bulk rubbers have a lot more rubber in them than typical screw-in rubbers).
Re. barrel length - this is probably the most important consideration. What is the visibility like where (& when) you dive, i.e. how far can you see in the water? This will help determine the length of speargun you will need. I would have thought Goa had gin clear tropical waters - but perhaps I am wrong. If the waters are very clear you might consider a longer speargun: 90/100/105/110/115/120cm. If I was going to Goa to spearfish and had no more info. than the images I just looked at on Google, then I would ideally want a 100-115cm speargun for reef fishing from shore and perhaps longer if fishing from a boat (e.g. 110-150cm). Under such conditions, a 100/105cm speargun might be quite reasonable for a beginner.
Some things to consider:
From a weight/hydrodynamics (resistance) perspective/inertia you want as small, slim, light & short a barrel as possible.
For power and range you generally want a long barrel.
For powerful spearguns, you might need some weight & width towards the muzzle resist recoil kick in the vertical plane.
Longer spearguns/stronger rubbers are typically harder to load. Double rubbers take longer to load.
Spearguns are sized by the barrel length, the spear will typically stick out about another 40cm beyond the barrel and the handle, etc. at the back end might add another 10cm to the overall length. Something to consider when shipping or planning to take a speargun on planes.
The Rob Allen railguns lean heavily towards the powerful, heavy, bulky end of the speargun spectrum. It is what I started with but I now use super-light, super-slim Omersub XXVs - because we don't often get huge fish here in the UK and the XXVs should cope with anything I am likely to encounter. There are many options in between.
