According to TolkienGateway the Towers of Thangorodrim were the "highest peaks in Middle-earth". Gonna look for an original source on that. In my personal opinion, the descriptions of Thangorodrim sound very volcanic.
Edit:
- Of the Return of the Noldor, The Silmarillion
Now of course "towers" can mean literal towers, but Tolkien's descriptive style makes use of metaphors like this all the time so my bet is he means mountain peaks.
- Of Beren and Lúthien, The Silmarillion
To be seen from that distance the Towers must have been exceedingly massive. Plus, the mere fact that they are used as
the landmark of Angband and that whole region implies hugeness.
You've got to remember that The War of Wrath (as with the whole ending stage of the Quenta) was left as practically just an outline. Nothing in the War of Wrath is described in detail and the whole section is overall underwhelming for events so world-wrenching and spectacular.
True, but in the case of Thangorodrim it seems clear to me that Tolkien meant the Towers were literally broken, as in many places in The Silmarillion there are references to the "breaking" of Thangorodrim and Thangorodrim being "broken". In fact the Towers are described as "broken" shortly after Ancalagon is slain.
As for whether Eärendil could've slain a mountain-sized dragon, it's important to keep in mind that Tolkien's original conception of his mythology was particularly mythological, in that certain details are fantastical to the point of defying logic, as is the case in much of real-world mythology. Doing the impossible is typical for figures of legend.
This is not the only instance where interpreting the text literally in terms of a "real" world seems impossible. For instance, the slaves and captives of Angband are only freed
after Angband and the Iron Mountains are destroyed (the mountains are "unroofed"), which of course makes absolutely no sense as they would've all been crushed.