It will actually be half an arch, because two mirror-image pieces will
comprise the full arch (spaced apart slightly). Each half will be about
24" long.

Imagine the board laying flat against the wall. I'll describe the
left-hand piece. The bottom will be straight (24") and right-hand edge
will be straight (6") and the arched top will curve from the top right
down to a point on the bottom left. There is no left edge because the
arched top meets the bottom edge on the left. The arch is quite
gradual.

Tom