You can already do this, at least if you can accept your setting applying to all windows. The method is undocumented, however. Close mIRC, open mirc.ini in a text editor, locate its [options] section, and in that section, find the 18th item on the n7= line. It should default to "70" (which means 70%). To achieve the approximate size depicted by your image, change it to "90" (for 90%).
I don't know why that option exists in mirc.ini yet isn't controllable via mIRC's GUI. Design oversight? (Another also-undocumented width option: item 29 on n4= under [options] controls the default nicklist width in pixels. It defaults to 0, and will affect any windows not already defined in [nicklist].)
http://www.mirc-support.de/help/mircini/