Perform "Undo", and do the thing from the very beginning. Radius and axes could be adjusted after action without Undo.

You can adjust radius from action menu, from action menu which is on bottom left side of your layout.

First step: you go to edit mode.
Second step: you place 3d cursor to place where arc center should be.
Third step: you choose which vertices should be aligned.
Forth step: click "Make arch" from edit menu
Fifth: adjust radius to desired size or select proper alignment axis.