I downloaded the 4 Gb version of 2.3.5, installed it on a CF en put it into a Watchguard 550e.
I got an error with this script, I think it is designed for hdd, so you have to exit the script and change it, see: https://forum.pfsense.org/index.php?topic=130307.0
Another issue is /var. ClamAV is using that for installing the virus definitions, but /var is too small and so it's inpossible to install ClamAV, but the CF is 4 Mb that must be enough….
Please have a look at these issues.