The reactor is stated to be around 100,000 years old, giving us a date for the creation of the sphere. The size is established when they talk about going down to the centre, and one of the Moneans exclaims that it is over six hundred kilometres deep. Technically this makes the diameter more than 1,200 km, which is why I tagged that one as approximate. The graphic supporting Paris's description of how it was formed indicates that virtually every  drop of water was taken off a planet to make the sphere. This - amazingly - works fairly well; if the sphere was distributed evenly across a similar area as our own oceans cover it would fill them to a depth of about 2,365 metres, compared to our the real average depth of about 3,600 metres. So the sphere is well within the probable variations in planet size and ocean area and depth. Don't ya just love it when the writers get it right!

The mass I calculated assumes a normal density of water of about 1,000 kg/m³. It's marked as speculation because for all we know the reactor unit in the centre contains a few gazillion tons of neutronium or something. The 80,000 population is stated by the Monean bureaucrat in direct response to a question. We know little of the extent of the facilities within the sphere, but they seem fairly extensive - a few miles square and maybe a couple of thousand feet tall at a guess.

We are given no indication that the sphere has weapons of its own, and I would guess not. The three fighter-type ships we see at the beginning attacking Voyager are presumably not the only ones around - the Moneans "shoot first ask questions later" attitude is explained by their having had to defend the sphere against outsiders in the past. I would guess that the place is fairly heavily armed, at least by their standards, but the specs page relates to the sphere itself rather than the craft it carries. I also can't see the containment field being much of an issue. Voyager fired a photon straight through it to intercept Paris's missile, and while they might have had a firing window cleared for them - although there is nothing to indicate that this happened - Paris himself had no trouble flying the Delta Flyer through the field and he most certainly did not have authorization. Although I suppose that a few hundred kilometres of water might count as armour, it doesn't seem to do squat against the weapons used in the episode; the Delta Flyer even fires phasers while its down in the depths! As a result, it gets a zero in armour as well. Since I can't see the sphere getting a rating for speed or agility either, it comes out at an overall value of zero - the only entry on the entire site that does. Technically this would mean that any old shuttle with a machine gun could destroy it, but realistically the size of the thing would make it tough to destroy totally. And the central reactor may or may not be armed and defended, of course.

The Moneans say that they are harvesting oxygen and food from the sphere. Later, it is established that its continued existence is threatened because the water is getting denser. This seems to be increasing the pressure on the central support unit, and as a result it is cutting containment field power and redirecting it to protect itself. Just how one makes water significantly denser by removing oxygen and fish is a mystery which I leave it as an exercise for the reader. ;-)