Probably 13mm or 17mm - but Steve Brotherton is the person to ask. Steve probably does more of these in a day than most of us will do in a lifetime.
I bought my flare nut wrenches in a kit - 10mm through 19mm - $15 or so. Pep Boys or Kragen, but easy to find at most auto parts stores. You use these rarely and on hydraulic fixtures which need little torque, so the the expensive, high quality ones are probably overkill, in my opinion.
J C Whitney has a set for $14.99 at http://www.jcwhitney.com/product.jhtml?CATID=5408&BQ=jcw2