Finder's column view can be a painful experience, which is why I seldom use it. I find if I resize the window by the bottom right corner (like all us old-timers grew up doing), the preview is on a one-way trip to thumbnail land. If you increase the window size, you get extra columns while the preview stays the same size. But if you then decrease the window size, it's the poor old preview that bears the brunt and shrinks in size. What a crazy state of affairs.
A far easier way of previewing files is to just use Quick Look… Select one of the files and press spacebar. You can reposition and resize the preview to your heart's content, and the preview window updates as you navigate through your files.
If you really want to use column view to preview files in the same window, here's how you can resize the preview:
- Resize the window down until you can only see the desired number of columns (minus the width of the preview column).
- Scroll to the far right so you can see the preview again.
Hover the cursor over the right edge of the window until you see the column resizing icon. It has a vertical line in the middle and two arrows either side…
You need to approach the edge of the window from the left. If you approach from the right, you'll get the window resizing icon instead (two arrows with no dividing line).
- Once you see the correct cursor, click and drag to the right to resize the preview column, thus enlarging the preview image.
It's not pretty but it works. (Hint: Use Quick Look instead!)
Oh and by the way, we'll be getting another option to preview files in macOS Mojave called Gallery View. It looks like it replaces Cover Flow. While it takes away the list of file names, it does add a metadata panel for the selected file which could be quite a useful little feature.