This is not the fault of mIRC, but rather how minimum/maximum window sizes work in Windows. When you make a window with min/max and close buttons in the titlebar, the minimum window size will be the width of the icon + the width of the buttons-- this ends up being something around ~114px wide.
If you need a smaller window, remove some of the titlebar buttons and/or remove the border entirely. For instance, using the "tool window" border-style allows a size of 60,60 to work:
//window -dpf +L @x -1 -1 60 60 | echo -s $window(@x).w
Will echo 76 for me (16px of which is the border size here, which I think is the default)-- you can see this is clearly lower than your reported values.
As a sidenote, this also has nothing to do with the -f switch. The same command as above without -f should also return the correct value (in this case, 60, since the specified size included the 16px border space). Note that without +L the window will be made to 124px, not 60. This shows quite clearly that the minimum size behaviour is not related to the -f switch.
