If you make the prism longer, volume=cross-section area times length. If A is the cross-section area then the original volume = xA where x is the original length and the new volume is yA where y is the new length. The ratio of the volumes is V1/V0=yA/xA=y/x, so you can find the new volume V1=(y/x)V0, where V0 is the original volume.
Surface area (original)=2(xW+xH+WH) where W=width and H=height (unchanged);
new surface area=2(yW+yH+WH), so new/old=(yW+yH+WH)/(xW+xH+WH).
This can be written: (y(W+H)+WH)/(x(W+H)+WH). You need to know W and H or their sum and product to see how the surface area is changed when the third dimension changes from x to y. Note that all 3 dimensions are interchangeable so it doesn't matter what you refer to as length, width and height. W and H simply represent the unchanging dimensions.