Yes, Paramo sizing can be a bit variable to say the least. Over the past 15 years I've had (still got) four of their smocks or jackets, 2 large and 2 medium. I'm 6ft tall and around a 40" chest and find that I take either M or L for virtually all outdoor brands.
Earlier this year I tried on several current models in a shop and found that to get a good fit i took a Large in the Velez Adventure light smock, a Medium in the Halcon jacket, and for the Enduro, their recent top-line mountain jacket with stretch panels, I needed an Extra Large to even get it to zip up, by which time the sleeves were ridiculously long on me.
So it really is going to depend what model you're interested in, and there's no real substitute for trying them on.
An interesting looking route through some lesser-walked areas. You must have covered some pretty big daily distances at times?
I'm familiar with some of those areas, especially the early and later stages...
Boulder field and bog on the way to Aberglaslyn (on ground to the south of Moel Hebog?) sounds about right. But the Glaslyn is always spectacular. The Cynfal gorge is a magical place, and some of the hills around Llan Festiniog are pretty wild and empty.
I know the Mawddach trail and the Creggenen hills as much from cycling as walking. After that your route's unfamiliar to me until you hit Llangynog and come up to Llangollen (did you pass through the Cerriog valley?) and on to Mold, which is our local stomping ground.
I'm not surprised you saw numerous birds of prey. We regularly see large numbers of buzzards, kestrels, red kites (and others we're less good at identifying) in those quieter hills and moors - far more than we ever spot on trips to mainstream Snowdonia.
It sounds to me like you could be talking about "Climb to the Lost World" which was a 1973 expedition to climb Roraima, by Hamish McInnes, Jo Brown, Don Whillans and Mo Anthoine. McInnes wrote a book of the same name, and A BBc Scotland camera crew went along to produce a film. I don't think it's ever been released on dvd.
I was lucky enough to hear Don Whillans describe this expedition as part of a slide presentation in 1983. He was a brilliant storyteller as well as a superb climber. Sadly he died just a couple of years later.