This question asks about how the elders were seated in the Lishkah. Seeing how poorly-sourced the OP’s dimensions were, I thought I’d ask it myself.
We’re told that the Lishkah was halfway in the Azarah (Yoma 25a), and that it was on the south side of the Azarah (Middos 5:4). But as far as I can tell, we’re not told more than that. I tried gathering its dimensions from the preceding Mishnayos in Middos, which place the Azarah at 187 Amos from east to west and 135 Amos from north to south (5:1-2), but those same Mishnayos seem to use up all of those Amos in the Azarah itself.
Are there any sources that explicitly tell us how big the Lishkas HaGazis (or the Lishkos in general) were, in the second Beis HaMikdash? I’d prefer a Gemara that I’m missing, but I’ll accept anything as late as the Rishonim, as long as it’s a precise number, not a guess based on a popular illustration or tile counting on an artist’s rendition.