I read some news about this new comet and some unreliable sources said it is twice as big as Jupiter. I tried to find a reliable estimation of its size or mass, but I couldn't find anything. Would you please provide me a general way to distinguish reliable facts from unreliable facts about comets? Like their size and mass range, how much mass they lose, etc.
The confusion comes from the difference between the nucleus and the coma.
The nucleus is a small icy body, only a few km across.
The coma is the cloud of gas and dust released from the nucleus as it warms up. With not much gravity, the coma spreads out into space, and it can be hard to say exactly where the edge of the coma lies, however, a coma "the size of Jupiter" would not be unusual. Active comets can have comas that are larger than the sun.
But the mass of the coma is tiny. It is made of a very low-pressure gas. It is nearly all empty space, most of the mass of the comet remains locked in the nucleus.