I'm going to say that Myles Parish is 5 foot 5 inches tall.
Although Myles is really popular on Vine and has thousands of fans all over the world, there isn't too much info out there on the internet about how tall he is.
Luckily, he hangs out with some VERY famous people, such as Arianna Grande, and there's plenty of info about how tall she is.
So if you take a picture of the two of them hanging out, and try to estimate the difference in height, then considering Arianna is 5ft tall, I would have to say Myles Parish looks about 5'4 or 5'5.
What do you think?