Therefore, the total length of the new wall is l = 2x+450/x (x > 0).

l ' = 2-450/x ^ 2。

Let L'=0 to get x x= 15, and x > 0,

∴x= 15, then when x= 15 and Lmin=60.

∴ The length is 450/ 15=30 (meters).

Therefore, when the yard is 30 meters long and 15 meters wide, the materials used for building walls are the least.

So the answer is: 30m,15m.