The episode Juggernaut gives us most of the details. It shows the freighter in the same shot as Voyager, which is where I get the length of 1 kilometre from. That's just a rough estimate, and as usual I'd be happy to hear from anybody who has more official figures.

Juggernaut establishes that the freighter is carrying "twelve trillion isotons" of waste. In the real world the prefix "Iso-" simply means "same as", but in Trek the term appears to be a multiplier used in the same way that we use "kilo" or "Mega".

The DS9 Tech Manual claims that a photon torpedo with a 3.15 kg M/AM warhead has a theoretical maximum yield of 25 isotons and an actual yield of 18.5 isotons. Assuming that by "theoretical maximum" they mean that the M/AM reaction would release all the possible energy allowed by E=mc², this would make one isoton equal to about 2.7 Megatons.

However, the DS9 TM's numbers are contradicted by the Voyager "Scorpion" episodes. These establish that their photon torpedoes are rated at 200 isotons, rather than the 18.5 claimed for DS9's photons. It's hard to credit that Voyagers torps are over ten times more powerful than DS9's, and four times more powerful than the minimum 50 isoton power of a quantum warhead given in the book. Probably the simplest explanation is that the DS9 TM's figures are off by a factor of ten, i.e. the theoretical maximum yield of a 3.15 kg warhead is 250 isotons and the actual yield 185, with Voyagers torps 200. This would make an isoton equal to about 270 kilotons.

Even using this lower figure, we get a number for the mass of waste carried by the Malon freighter in "Juggernaut" of  over one million trillion tons. Using 1 isoton = 2.7 Megatons would give a cargo mass of ten times this, or ten million trillion tons!

The waste we see emitted by these ships is always in the form of a gas, but if the mass carried is indeed of such a high magnitude then it's hard to believe that they are carrying tanks of gas around. I'm assuming that the substance is stored as some sort of very high density solid, which changes to a gas as it is released from the storage tanks. You could maybe argue that the gas itself is immensely dense - super massive atoms, perhaps - but whenever we see it released on the inside of the ship it looks only slightly denser than air (almost as if had exactly the same density as CO₂, snigger snigger). Of course, that assumes that the stuff we see released on the inside is the same stuff we see released on the outside.

One other possibility is that the Malon guy is not in fact referring to the mass of the gas with the four million isoton number, but rather to its explosive yield. This would avoid having to have super high density storage, but it doesn't really fit the dialogue well :

Fesek : "Listen to me very carefully. The fact that we're still here means the ship hasn't exploded. Yet. When it does, over four trillion isotons of antimatter waste is going to ignite. Everything within three light years will be destroyed."

The fact that he is directly referring to how much waste will ignite seems to be a fairly concrete sign that this is mass, not yield. But if the above demonstrates anything it is that whatever interpretation you make is loaded with assumptions.

The crew numbers are based Feseks statement that when the gas leaked over sixty crewmen died within minutes and "only a few of us made it".

That the freighters are armed with Spatial charges is established in "Night", but the lack of them since this episode indicates that at least some freighters are not in fact equipped with these weapons. The power of these devices seemed quite impressive, and I have rated them accordingly.

An very minor but interesting nit - Fesek refers to "freighters of this gradient" in the episode. I think he must have meant "grade", as in the quality or class of ship. In the US people refer to a slope as a "grade", short for gradient, and I suspect the actor or writer substituted the word by mistake.