If you want to maintain compactness as in "bonsai," roots need to be cut. Your 15" pot would be okay as long as there is plenty of aeration in the media to allow for good drainage and some time for top layer of media to dry between waterings.
For the 13" depth of your 15" pot....to solve the issue with depth after cutting roots, place gravel at the bottom of the pot, place a screen material on the gravel, add an inch or two of media on top, then plant your adenium and fill sides in with media.
