ZBrush has always been a full-screen app on the Mac.
I’m running OSX 10.8.5 and I can resize Zbrush window just fine. It’s a little tricky sometimes but it has to be done in the lower right corner. The mouse icon won’t turn into the diagonal adjuster but should turn into the width or height icon arrows. Then just drag and resize.
As for the OP working in graphics and 3D it’s extremely useful and more productive to have a multi-monitor setup. Even if you get a cheaper secondary monitor it’s well worth the investment. Until then you should be able to resize from the bottom right corner.