Let the width be W, the length be L, and the height be H.
total length of all edges = 4(W + L + H) = 40 --> W + L + H = 10
total surface area = 2(WL + WH + LH) = 48 --> WL + WH + LH = 24
We want to find sqrt(W^2 + L^2 + H^2)
We can use the following algebraic identity:
(W + L + H)^2 = W^2 + L^2 + H^2 + 2WL + 2WH + LH
100 = W^2 + L^2 + H^2 + 2(24)
W^2 + L^2 + H^2 = 52
The space diagonal's length is therefore \(\boxed{\sqrt{52}}.\)
.