First, do you have set in Windows 8 a custom display wich allows to change the size of all items ? This following image show the Windows 8's "Change the size of all items" dialog box.
If you're not at "100%" (green circle in the picture), all items will be bigger and more easier to read.
However, setting a ratio wich is different from 100% may causes some graphical's problems on many applications. Windows 8 and some others applications (like firefox) supports practically any kind of ratio (ex: 125%, 150%), why ? Because these applications/OS use for their graphical's elements standard's windows control (button, editbox, listbox, toolbar, etc.. wich can be themed ).
Paint with "size of all items" set to 150%. Everything seems correct, all the elements seems equilbrated:
For his Toolbar, mIRC seems to uses his own toolbar's control, it's not a standard windows's toolbar. With "size of all elements" set to 150%, the mIRC's toolbar will be smaller than others controls present in mIRC:
Changing the option value between small, medium and large does not do anything
It's normal, this switch resize the mIRC's switchbar (contain servers, channels, query, etc..) and not the mIRC's toolbar.
Maybe someone will correct me, but I don't think it's possible to make the mIRC's toolbar bigger or smaller.
Add this feature to mIRC would be nice.
However, if you want to equilibrate all the mIRC's control I advise you the set the size of all elements to 100%.
If you're already at 100% and the toolbar is too small, I don't have an answer. In this case, please sends more information about your problem, and post a screenshot.