What is the length of the side opposite the 30 degree angle?

It depends on the length of the other two sides which creates that angle. Not enough information was given.

However, you can simply use the Cosine rule to find it if the other two lengths are known.

a2=b2+c2- 2bccosA

(A=30 and a is the length of the side opposite to 30 degree angle; b and c is the length of the sides which makes up the 30 degree angle)