Option to use one window size for every file

I filed this thinking Mud didn't remember window size. It does — per file,
since 4.0.0 — so my original description was off. Rewriting it to describe
what I'm actually after.

Per-file sizing works nicely if you want different documents at different
sizes. I'm the other sort: I keep everything filled to the screen. With
per-file memory that means filling each document the first time I open it, and
there are always new documents, so it never quite settles.

What I'd love: an option to use the same window size for every file — the
size of the last window I sized — as an alternative to a size per file.
Per-file would stay the default; I'd just switch it once and forget about it.

A smaller, separate thing: a file Mud hasn't opened before appears at a
fixed 860×740 rather than the size I last used. That one might be worth
changing whichever mode is on, since there's nothing to restore for that file
and the last size I chose is a closer guess than a constant.

One incidental benefit, if it appeals: in "same size for every file" mode Mud
would only need to keep one window frame, rather than one per file opened.

I've had a go at writing both up as a plan document, in case it's useful —
I'll add it in a comment below. I realise the first part is a product call
rather than a bug, and I'd completely understand if it's not something you
want in the app. The second part stands on its own either way.

No files or tests are named. Start by locating Mud's existing per-file window-sizing behavior, then clarify whether the shared-size option and the first-time-file fallback should be handled together. Done means the requested sizing behavior is implemented with per-file sizing still available as the default.