The general rule is 10%-15% of their body weight or a bit bigger than the widest part of the body at that age. With the right set up it should get up to size fairly soon. Make sure temps and humidity are on point and you'll be fine. Nice normal BTW. Pastels have green eyes so that's something to look for in the future. If it was sold to you as a Pastel I'd talk to the store where you got it. Likely you were over charged, even at store prices.

Dave