How did 9 hikers, potentially a maximum capacity for 10, fit into that 12x6 floorspace? However they did so it must've been very cosy.
The Russian TV series
Pereval Dyatlova showed the hikers getting ready for bed and they were laid out in 3 groups of 3 along the tent length. I wasn't sure if that was possible and whether it would require them to be 4ft midgets or they'd need to sleep on someone's lower legs and feet. But with 3 across the 6ft wide tent there may have room to slot in.
The book
1079 The Overwhelming Force Of Dyatlov Pass includes a graphic which suggests all hikers slept with their widest parts, their shoulders, to the same side of the tent, and all were flat on their backs bar Zina who was laid on her side. Even the 6-footers were laid flat across a 6ft wide tent, which would push their heads and feet into the canvas, and some hikers extended their effective height by sleeping with their arms above their heads, which would also tend to bare their hiker armpits to the person next to them. I wasn't sure about this either, even more so when the graphic includes a pile of firewood a person-wide in the middle of the hikers.
No data exists online for the Soviet Union, but across the world biacromial diameters, the measurement from the centre line of the shoulder joint to the opposite side (not including muscle and skin beyond these edges) has increased since the 1950s due to higher calcium and protein intake. The Soviet Union's diet vastly improved from the 1970s onwards through major farming advancement but prior to this they may have had less developed skeletons.
I'm a '60s kid, and am 20 inches across the shoulders, skin-to-skin, possibly more than average. Place 9 of me in a tent, including one of my side, heads & shoulders all to one side, and even when packed in like sardines the tent would need to be 14.58ft long, and that is without any firewood.
1960s Americans were said to be 15.6 inches for men (bone to bone) and 13.9 for women. Allowing for muscle mass and the outside edge of the humerus, let's add an inch either side. So 7 men x 17.6 plus 1 woman @ 15.9 and one on her side, call it 13 = 152.1 inches / 12 = 12.68ft. It would very be tight, this is shoulder-to-shoulder, and in fact their elbows are slightly wider and would jab into each other, and definitely no room for any firewood, or a tenth person in this arrangement.
https://www.healthline.com/health/average-shoulder-width#1960s-u.s.-measurementshttps://www.cdc.gov/nchs/data/series/sr_11/sr11_035acc.pdf (the index page is wrong and biacromial measurements are on page 30)
Of course Soviets may have been narrower, although working against the averages this was a bunch of athletes. One thing is for sure they would have been tightly packed in with hardly an inch to spare, yet the book uses a graphic which depicts a tent 4 times as long as it is wide, equating to 24ft, which shows them with plenty of room between them.
So how would the hikers sleep in this tent, would they top & tail (which risks smelly feet either side and the potential for being kicked in the mush during sleep), or would rather more of them sleep on their sides, which I tend to think people will do for a variety of reasons (such as when on an uncomfortable floor, in a colder environment, and because most people automatically roll from side-to-side in sleep to avoid pressure sores and blood clotting), or would they bunch up and get cosy in some other arrangement, not exactly like the rescuers shown below, who I think were larking about after playing cards, but something as inventive in a cramped space.
https://dyatlovpass.com/resources/340/gallery/6S-02F.jpg