I like to think this.
- I think a regular mare is 4 feet 5 inches, more or less because not everyone is the same height.
- I think a filly (the cmc for example) is about a foot shorter, at around 3 feet 5 inches, a little more or less possibly.
- Fancy pants, Luna, Fleur de lis, Sassy saddles, Cadence, and other ponies like that are probably around 5 feet.
- I think celestia would be about 7 feet including her horn.
I also made a picture and journal explaining the height of wings and horns. If you want to read the whole thing here is the linkhere is the link. This shows how big wings and horns are, while this onethis one shows how big the ponies themselves are.
Just to shorten it, a filly's horn is 5-7 inches, an adult's around 9 inches, Fleur, Luna, Cadence, Fancy pants etc. have a horn of around 14-16 inches, while Celestia's is 21 inches. For the wings, a filly's is a little less than a foot at about 10 inches. For an adult pony, wings are 27 inches or 2.25 feet. Luna and Cadence's wings are around 33 inches (almost 3 feet). And Celestia's wings are a whopping 45 inches, which is almost four feet.
Hope this helped!