Add opera:config back and in opera:config under the Mail section, adjust the "Mail View Two Lined Messages" setting. 0=1 line. 2=2 lines all the time. 1= 2 lines when list and message on right layout is set.
Since opera:config is just a font-end to operaprefs.ini, you can close down opera and edit operaprefs.ini in the preferences folder (see help -> about Opera mail) instead. Under the [Mail] section (add it if it's not there), add/set the setting to what you want.