Is this answered in #689?

Using Typst' measure and scale:

#let c = cetz.canvas({
  import cetz.draw: *

  circle((1,1), fill: red)
  content((5,0), [Some test text is here. \ What do you think?])
  rotate(280deg)
  rect((1,3), (rel: (2, 2)), fill: blue)
})

#place(context {
  let (width, height) = measure(c)
  scale(10cm / width * 100%, place(c))
})

I think you just hardcoded the size of the container in that 10cm? That needs to use an invocation of layout I think. Doable by itself, but it's misbehaving in other ways:

Using place makes the following content overlap the drawing. They seem to be doing something to the layout though, removing both instances of place makes the drawing shift to the left, inside the left margin.

Adding float: true to both place makes the following content not overlap the drawing, but now I've lost control of where in the text the drawing goes -- float moves it to bottom of page.

Ah, it's the scale that makes the drawing poke into the left margin. scale(origin: left, makes it work correctly.

Here's what I now have:

#let diagram(canvas) = {
  context {
    let canvas-size = measure(canvas)
    [canvas width=#canvas-size.width]
    layout(size => {
      [parent width=#size.width]
      scale(origin: left, size.width/canvas-size.width * 100%, canvas)
    })
  }
}

#diagram(cetz.canvas(
  {
    import cetz.draw: *

    content((-5,0), [left])
    content((5,0), [right])
    }
  ))

The remaining annoyances are that just simple scale changes font sizes and stroke widths. It'd be much better to inform the canvas of the area it has available, and let stroke widths and text sizes like 8pt stay as 8pt.

You can also try the first option (without scale), I've mentioned in the linked issue. The problem is, content might not resize the bounds correctly.