I have this feeling that we'd need an über-nerd - even an über-Alcuin, basically Christopher himself or someone who has dug into UT and HoMe, perhaps using CoH, B&L and FoG as compact collections of all JRRT wrote about his three Great Tales, to distill how much "fodder" there is for any hypothetical Sil.Or I could wait and let someone else check
I could well imagine CoH, B&L and FoG as stand-alone books at least of the size of TH, going into detail. Though when one considers that in UT's "Of Tuor and his coming to Gondolin" JRRT spends 34 pages just getting Tuor past the last gate of the secret way, to get a first far glimpse of Gondolin itself, all bets on the possible size of that book are off to my overwhelmed imagination!
And JRRT himself might have also produced a "Lays of Beleriand", having finished "The Lay of the Children of Húrin" and "The Lay of Leithian" (but he mentions so many other lays in his writings that his "LoB" could have become a very fat book to include all, or at least the major ones).
Somewhere, recorded in some book on the legendarium, Christopher (I believe) confirms the impression that some people had gotten of the Sil published by him of being quite compressed - I'm almost tempted to say a "Reader's Digest" of the First Age (and the Second), with the long stories, the Lays, and a collection of annals being the source material. And that this was his father's intention for the Sil.
From that perspective a LoTR-sized Sil looks highly unlikely.
But as we know, none of the Lays even attempted were nearly finished, nor any of the longs stories. Quentas may have been finished for an earlier stage of the legendarium, but were all insufficient for post-LoTR Middle-earth, and their subsequent expansions and alterations did not get finished, IIRC. Not sure if any of his many annals ever reached that stage, but the various versions certainly needed reconciliation of contradictions. And never mind that more (far more) than once, JRRT seems to have had an inspiration for an entry in those annals that led him to leave the annalistic mode and produce a major essay, on the topic that had fired his imagination. "Unmethodical and dilatory" C.S. Lewis once called JRRT - these two long-time friends seem to confirm the saying of finding a splinter in the other's eye while ignoring the beam in their own … Or to go to our "real world", that inscription on the Temple of Apollo at Delphi - the oracular site in classical Greece - "Gnothi seauton", meaning "know thyself", is something our species is so pathetically incapable of (and the Internet definitely ain't helpin' … )