Since the cars aren't "scale", 1/43 and even 1/48 are a little large. S is too small. The issue is that the proportional size of children isn't the same as adults. Kid's heads are much larger than the rest of their bodies. I can find figures the correct height but they look off because their heads are too small.
Some of the Walmart figures are OK for platform use, especially the adults and the younger children (year one or two). By the time you get to Deathly Hallows, the figures are larger than some of the adults.
BTW, the figures are nicely done and worth the price. They are intended as collectibles, not scale models per se. Each one is to scale, it's just that they are all different scales
My daughter and I toured the sets at the WB studios in England. It is surprising to see a) level of details on the set pieces, b) the size of stuff is all over the map. While Hagrid was played by Robbie Coltrane (face/voice) in most scenes it is an incredible (and creepy) animatronic head on a a very tall actor wearing a Hagrid suit with the "puppet" head on top of him. Closeups and speaking scenes are of Coltrane (he is 6'1" but the character is supposed to be closer to 10' tall).
The main castle model is 1/24 scale. It's also two stories tall and over 50' in diameter including the docks, rookery and the "landscape". Even if we shrunk it down to 1/48 scale, the castle's foot print wouldn't fit inside of my two story 1800 square foot house.
video tour of the model
https://www.youtube.com/watch?v=XvgngWlr1ro