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DCX.dll FAQ (outdated): http://www.tg007.net/dcx/dcx.htm
DCX.dll Download (latest): http://www.tg007.net/file-1496

- Thanks!