Find out the dimensions of the sky couch, and make a mock up of it using the floor, couches and a coffee table (or anything that won't budge easily).

We did this and it was very accurate and we had no surprises when we got on the plane.

The only problem you might have is if the people in front of you recline, it does take away a bit of space. Hopefully you get other sky couch customers in front of you and no one reclines

We are both short and slim (170cm and 160cm) and while it was a snug fit, we managed a few hours sleep. Any bigger and it would have been a struggle.

We decided on the second flight that it would be easier if one of us sat upright while the other slept (with their feet on the other persons lap) which worked out quite well and we both got 4-5 hours sleep each on the 12 hour flight.

We have not flown PE before, but we would get the sky couch again on a longhual overnight flight if we had the option.