I've been mostly working on very large modeling projects (think detailed architeture over 1000 acres) or animation because this really was the "new" capability area that Opus 5.5 "unlocked".
I have been surprised how well it has worked at scale with many sub-agents. However the code base is limited to the rendering engine mostly.
Certainly in the older models they hit a complexity limit, but I haven't really hit it with Opus 5.5. I have had it make data flow designs that I didn't like because they were more complicated than it should have been or the perenial problem of trying to make everything backwards compatible when working on greenfield refactors, but I haven't seen it choke on a code base yet.
To my experience, it's a simple complexity curve where the first bits go fast and the speed goes down very fast once you reach a certain level of complexity. Size doesn't matter as much as complexity: with the right foundations, it keeps zooming through it. Now I do wonder how much my inexperience in the complicated project is a brake on progress vs. it just being a hard to tackle problem.
On the other hand, I have a TS only project core is sub-20k LoC (NES harness), where it is struggling (for now, see other comments)