The main issue with NAT'ing twice is protocols that are NAT-unfriendly. That includes some VPN client software, some VoIP protocols, FTP, amongst others. These protocols are a pain to deal with when doing NAT once, adding a second NAT into the mix makes it twice as difficult to make these things work right and troubleshoot when things aren't working.
It should be avoided if possible, because it's usually adding a layer of complexity that's unnecessary. In your case, I would see if you could use the modem as strictly a bridge and put the static IP on pfsense.
It doesn't affect packet size because NAT changes the source IP and possibly port (depending on the NAT implementation) on packets, it doesn't add anything to them.