I did a couple searches, and there were a couple of old necro threads, but they ask if I want to respond from 2021 or start new, so here goes...
It seems that Catalyst is using 1:265 for the current plastic mech miniatures. I've measured the Atlas sculpts from the Alpha Strike Box and the Inner Sphere Direct Fire Lance Force Pack. They are 57.5 mm and 56 mm respectively (+/- 0.5 mm in each case due to parallax, straight edge positioning, etc.). Also, the force pack one is sort-of walking, so maybe that accounts for about 1 mm. This gives a range of 15.2 to 14.8 m. Not bad. At worst, it's within only about 0.2 to 0.6 m at 1:265, but of what?
Battletech Universe states the height of the Atlas is 15.4 m. So, after all the years of debate, we now know the height of the Atlas. It's 15.4 m. End of. I was told that later fiction trumps earlier fiction, so if it was stated differently earlier, the Atlas is now 15.4 m. That's very close to 1:265 scale with the miniature and ties in. I can buy it, both from the miniatures AND Battletech Universe. It seems incontrovertible that the Atlas is 15.4 m at this point. At 56 mm, the scale is exactly 1:275. At 57.5 mm, it varies from 1:265 by only 1.1%
Here's a challenge, though. Battletech Universe also states the Visigoth's wingspan is 12 m, yet the miniature's wingspan is 67 mm. At 1:265, that's 17.8 m!! There's a discrepancy. It's worse, because in Battletech Universe, the Visigoth's drawing is scaled so far down, that it looks like the 1.7 m human depicted can't even fit in the cockpit! What happened there?
So, are the mechs scaled to 1:265? The Atlas miniatures and Battletech Universe together suggest yes, and the height of the Atlas is 15.4 m.
Are the fighters also scaled to 1:265? Not possible, if trying to reconcile the miniatures with Battletech Universe. Worse, it looks like Battletech Universe flubbed the scale of the Visigoth, so now, can we also not trust the Atlas?
And, finally, are the current vehicles (in the Mercs Kickstarter) scaled to 1:265? They look good to me, but is that right?