Standard sizes are 160 cm x 50 cm OR 150 cm x 50 cm.
The typical fabric used in dakimakuras is usually a variant of 2way Tricot (which is itself a type of spandex). The most popular fabric variants in Japan at the moment are Lyctron (exclusively from A&J) and Aqua Veil (a slightly cheaper variant of Lyctron but slightly more durable from my experience).
As for the price, I think 75 USD would be reasonable for a 160cm x 50cm dakimakura. There's a site called Dakimakuri that partners with various artists and makes and sells the dakimakuras if you're interested.
And no, I am not affiliated with them at all. I saw their BOOTH shop and got interested because I was looking for some Xenoblade 2 Nia dakimakuras.