I have no familiarity with blue whales but I would guess they're 1--5 times the mass of lorries, which I guess weigh like 10--20 cars which I in turn estimate at 1.2--2 tonnes, so primitively 12--200 tonnes for a normal blue whale. This also aligns with it being at least twice as large as an elephant, something I estimate at 5 tonnes.
The question asks for the heaviest, which I think cannot be more than three times the normal weight, and probably no less than 1.3. That lands me at 15--600 tonnes using primitive arithmetic. The calculator in OP suggests 40--320.
The real value is apparently 170, but that doesn't really matter. The process of arriving at an interval that is as wide as necessary but no wider is the point.
Estimation is a skill that can be trained. It is a generic skill that does not rely on domain knowledge beyond some common sense.
I would say general knowledge in many domains may help with this as you can try and approximate to the nearest thing you know from that domain.
How you get good at being a generalist is the tricky part, my best bet is reading and doing a lot of trivia (I found crosswords to be somewhat effective at this, but far from being efficient)
No, that has nothing to do with it. Trivia helps you narrow down an interval. It is not necessary to construct a correct interval, which can be of any width.
I am dying inside imagining someone who had to use crossword puzzles to learn how to read. There must be a better way to educate the masses!