Yes, it's not an issue with CF card space nor, normally, with space in /tmp or /var which are the same size across all Nano installs. That said I had to increase the size of /var on my home box to 80MB to avoid errors a few versions back. Can't remember the details now. :-\ I have 512MB in the box, and could easily add more, so it's much less of an issue.
Edit: Yes here we go, anything in this thread look familiar:
https://forum.pfsense.org/index.php?topic=66588.0
I was running out of space in /var every time the RRD backup ran at midnight.
No, I'm not sure of anything really. ;) I haven't investigated the code closely but I seem to remember something reported by one of the devs to that effect. Maybe it's more of an issue when the compressed data is extracted.
Like I said moving data onto a USB stick would be a last ditch solution.
Ah, I see what you're saying about the modem interface. If it doesn't have a gateway on it that there won't be an RRD quality file created for it though.
Steve