What ships will the Origin 100 fit inside?

It will fit inside any that it physically fits inside. We didn't design it specifically to fit in or out, or not in or out, of any specific ship. It's broadly similar to the 85X in dimensions, so you could theorize that it would complement an 890 Jump nicely if you wanted to do that. But beyond that, we haven't gone out and measured...

I'm sure that people will find some ways to make some ships fit somewhere.

I've seen the videos of Starfarers laden with stuff, but compared to ships like carrier ships, where we have strict metrics for fitting in, say, an Idris or a Javelin, it wasn't as strict a priority to keep on top of that for this.