Meralygos

 To fill a few blank spaces. In this section I am writing lore I infer from World of Warcraft sources. Mind, there is not an explicit confirmation of my words -hence this is headcanon - not real lore!

 

According to the images we have seen in the comics and in the cinematics, dragons' dimensions vary a lot. An average young dragon - the size of, say, Tyrygosa or Kalecgos, is inbetween 30 or 50 feet, tail included (10-15 meters). Arguably, ancient dragons such as Onyxia or Korialstrasz look much bigger. From the comic, we know Tyrygosa is not as indimidating as other dragons size-wise. For example, Korialstrasz was much bigger than she was (note: Krasus is over ten thousand years old). I'd argue older dragons can reach about 70 feet, if not more! 

Average height of a dragon standing on two feet - about 20-21 feet tall [first picture below, Tyrygosa]

In the image below, Tyrygosa standing on two legs is about four times Dar'khan (with the neck bent, looking below). According to TBC atlas, a sin'dorei is about 5.5 feet tall. Tyrygosa standing on her rear legs is about x4 times Dar'khan: about 21 feet tall. 

Kalecgos looks slightly bigger. The second picture is a comparison Kalec-Anveena. She's not as tall as Dar'khan (I gave her about 4.5 feet tall, perhaps she's taller). In the picture below, Kalecgos is x8 times Anveena, making him about 36 feet long (with curved tail and deployed neck). Overall I'd say a young dragon like Kalecgos is about 38 feet in lenght, while bigger dragons such as Onyxia or Korialstrasz are much, much bigger, likely above 60 feet (18 meters).

The last three images show the battle between Tyrygosa and Krasus. As you can see, Krasus is far bigger than Tyri.

Battle: Krasus vs Tyri. Krasus is much bigger than Tyri!

A normal black dragon from the WoW comic. Compare the moment he tries to burn Valeera - his muzzle for example. He's about the size of Kalecgos and Tyrygosa.

Picture of Onyxia's size. Compare her to Valeera (the elf jumping around). She's much bigger than the above dragon. 60-70 feets most likely!