Axes are necessary in order to differentiate masters. Having an axis defined doesn’t automatically mean that you can use your font as a variable font, or that intermediate locations between the axis poles need to exist.
So, in your case, it sounds like two axes would make sense: One for roundness and one for counter size. Add two axes in Font Info > Font, name one RNDS and the other CTSZ (or whatever four-letter abbreviations make sense to you). Now, in your masters, pick whatever values make sense to you: for example, use 0 and 1 for both axes. Your first master would be RNDS=1 CTSZ=0 (round, small counters), your second master would be RNDS=0 CTSZ=0 (not round, small counters). Repeat the same but for the masters with the larger counter size (CTSZ=1).
This way, you will have four masters with unique axis values, meaning they each have a unique location on your axis grid. Again, this doesn’t mean that your masters interpolate (as in, they don’t necessarily allow for intermediate steps between each other), but you can still keep each master in the file and set up exports corresponding to each master. Simply set the same axis locations for each export in order to point to your desired master.