I presume you would like to have your channel windows maximised and at the same time want the channel topic, amount of users etc to remain under the 'buttons' and have the titlebar free to show whatever you want to have there correct?
If so, do the following:
1. start mIRC as per usual and join whatever channels you want to be in.
2. Next minimize all windows so you only see the 'background'.
3. Next click on one channel so you can see the window and click on the 'Tile' button.
4. Minimize the channel you have just done and repeat step 3 for any other channels until finished.
If you exit mIRC completely and restart up, your settings for the channels should be saved to how you just set them