The Blacketts
I get the impression that Nancy and Peggy are pretty similar to look at - they seem to be easily recognisable as sisters. Nancy is described as bigger than John in Swallows and Amazons, although they seem about the same age, and the size difference doesn’t seem to be referred to later.
If Nancy had been significantly older than John (more than a year say) I think she would have claimed Commodore-ship by right of age. Peggy is as big as John and by implication bigger than Susan, although, again, I get the impression that the mates are about the same age and definitely see themselves as a step ahead of Titty, Dick and Dorothea.
Girls tend to get to the growth -spurt phase sooner than boys - although having said that Susan is also a girl and this can run in families, so I’m also concluding that Peggy and Nancy get taller younger, but do not necessarily end up as taller than the Walker girls and Dorothea, since they might also reach their maximum height sooner. I do think they end up taller than their mother, and considerably taller than their Great aunt. So I’m picturing Molly Blackett as about 5ft - which is about as big as she could be and still be short enough for it to be particularly noticed by the Walker children. Nancy and Peggy end up as taller, presumably because their father was at least average height and possibly taller, but I’m not picturing them as taller than about 5ft 4 to 5ft 5, which I think would be about average height for their generation. I’m fairly sure that Nancy, whenever she wishes, can dominate regardless of height and somehow always appears to be taller.
I also picture them with more definite colouring than the Walkers. I’ve come across a few versions of Nancy or Peggy with red hair, but that doesn’t convince me. Not only does no-one mention red hair - and we do get a description of them through John’s telescope- but I think it’s unlikely that the Great aunt would want them in pink sashes if they had red hair. I know people with red hair who look very nice in pink, but the convention of the time and earlier seems to be that red-heads did not wear pink - and certainly not red knitted caps!
On the other hand, having brown hair, especially dark brown hair, seems to predestine a girl for RED KNITTED EVERTHING. I’m speaking from personal experience here. (And I do appreciate that it was kind of Granny to knit so much for me and maybe red was the best colour for being visible if I fell into water so I could be seen and rescued before I drowned. Not that I had that many chances to fall in water.) So, I’m picturing Nancy with dark brown hair and Peggy with hair that might have been slightly lighter, but was still dark rather than light brown. There is reference to Nancy being sunburned and also going red with anger - visible through the sunburn, so I’m guessing they both had quite fair skin - the sort that burns and then tans lightly, but doesn’t really tan properly and blushes very easily. To go with that colouring, hazel, green, grey or blue eyes might be quite probable, so I’ve given Nancy hazel eyes and Peggy blue and Nancy straight hair and Peggy wavy hair in my fiction.