You can't setup a circular print volume via MM's GUI.
But here's a workaround:
1. Create a custom printer in MM:
Let's say you put in "MyPrinters" as the Manufacturer and "MyDelta" as the Model. Define the PrintVolumeDimension were Width and Depth is its diameter.
Click Add to store this printer and close Meshmixer.
2. Now browse to your meshmixer directory (by default it is within your Documents directory). Inside meshmixer is a "printers" directory containing a "printers.xml" file. Open this file in a text editor. Using its search functionality search for "MyDelta". This should lead you to a section like this:

Edit its line
<BedShape>Rectangular</BedShape>
to
<BedShape>Circular</BedShape>
and save the edited file.
Now start MM and your printer should show up as a circular one:

Gunter Weber
Triangle Artisan