This has been annoying me since switching to Thunar as well.
The option isn't in Preferences, it's in the "Configure Columns" dialog (View menu). There's a checkbox at the bottom you need to clear.

Thank you very much!
I would have never discovered this myself...

I have a recurring problem with Thunar: I use "View as Detailed List", with only two columns: "Name" and "Size".

But it seems that the "Size" column very often ends up pushed out to the right, outside of the viewable area. Several times every day I have to scroll to the right, and drag the "Size" column back towards to left. I would really like a setting that will enforce that no columns can disappear off the viewable area, so that the sideways scrollbar will never appear!

I would rather like to have the filenames under the "Name" column to be truncated, than to have the "Size" column pushed "off screen".

Is this possible?

And another thing: why it is possible for the "Size" column to become arbitrarily wide? The number that indicates a file size is never longer than a few characters even  for extremely large files, due to the use of units, i.e. 9.5TB. When I drag the vertical separator between the file list and directory tree towards the left, it is the "Size" column that increases in width. Ideally, the "Name" column width should increase and the "Size" should stay the same (quite narrow).

Actually, I think I just found a workaround: put the size column on the left and name on the right... It seems to work much better :-) The size column now stays at a quite narrow width while I drag the vertical separator back and forth. Fantastic! Or perhaps I'll stop using the size column alltogether.

Best regards
Torquil Sørensen