Essentially, I need to be able to resize columns. ( I'd be happy to explain why - please let me know - but I'll spare you the detail for now. )
From this and this, I deduced that it's a feature scheduled for release in version 4.0.
So I looked for a prerelease, found one here, installed it, created a separate workspace for it, but the feature to resize columns is, apparently, not yet implemented in that version.
In OR 3.x a work around might be (depending on your exact need) to "collapse" various other columns so that the column that needs the extra width automatically uses the extra space provided (this is clearly not really a solution and you are still at the mercy of the way the column widths are calculated for you, but I've found it effective in some cases).
The option is under the column dropdown > View and you can do things like "Collapse all other columns"