You should be able to load your CMAQ file, which is in the IOAPI (quasi-netCDF) format. Unfortunately, there are times when VERDI doesn’t produce an error message.
VERDI does not have a RAM cap; it is only limited by the available physical memory in your computer. Just specify the -Xmx parameter in the VERDI run script to a value less than your available physical memory. You can definitely load model output files much larger than the -Xmx memory limit. The memory limit only becomes a factor once you start adding variables into Formulas for plotting; so after adding several 3-D variables, for example, you may need to limit either the Time Range or Layer Range or both.
Can you load any other CMAQ files, like those under ./data/model included with the VERDI distribution? A few more details on your CMAQ file and your computer system might be helpful.