Is there any rule-of-thumb regarding tree size and the size of pot/container they should be kept in?

Kind of depends on what you are trying to achieve....

Here is a link to an excellent discussion...

https://www.ourfigs.com/forum/figs-home/147321-productive-container-grown-fig-trees-what-and-how

It's just to keep them under control and not grow too big for my limited space until I get rid of some fruit trees that have never performed well - too shaded by my eucalyptus trees so they're going to be halved in height shortly.

Thank you for the link.

For some reason the link isn't opening.

Hmmm... Not sure why it isn't opening for you... I just checked it by clicking my own link and it opens... membership to that forum is not required to read the posts so that should not be an issue if you are not a member there...

Can't open the home page either. Mmmm.....

Links working today.

I have limited space but if my fruit trees don't come up with the good they're coming out and will be replaced with figs. Cutting down my eucalyptus trees would help a great deal too; they cast some shade, perhaps too much, but in summer I get over fourteen hours of full sun on cloudless days.

I'm going to try air pots with some of mine. I'm keen to try them.

when talking about fig trees or any other fruit trees there are about three growth habits:

1.Upright growth- tree prefers growing tall.

2.Sprawling growth-tree prefers a Gnarly shape, borwn turkey does that.

3. Somewhere in the middle between Sprawling and upright.

I believe there is no relation of pot size to the tree height especially when considering these growth habits.

If one could measure the bark area across all branches of a tree it would be a good measure of correlation to pot size.

My Brown Turkey outside my kitchen window is in a 40 litre container and is all over the place. As for the others I have, I hope most will grow upright but I'll take what comes and adapt. I also want to graft different types onto root stock to keep in my greenhouse year-round.