mIRC has not the ability to maximize an dialog so far, but if you want only to attach minimize and maximize (maximize not working) button on an dialog use
-md parameteres.
I suggest you to use DCX.dll to do this job.
DCX.dll FAQ (outdated):
http://www.tg007.net/dcx/dcx.htmDCX.dll Download (latest):
http://www.tg007.net/file-1496- Thanks!