I notice that the file icons are treated much like a column, in that when a thumbnail column is used the file icons disappear. The thumbnail column can be sized to any size from the folder formats dialog.
Using a 4K monitor the details view file icons are very small so another feature request 
To have a separate icon column in folder formats that will allow size to be set like the thumbnail column.
They shouldn't be small if you have set your Windows display scaling properly. (And if you haven't done that, almost everything in every program will be too small.)
Scaling my 2 monitors was a compromise due to the advent of Dopus on my machine. The 55" 4K OLED was comfortable at 100% scaling (used mainly for watching tv) and the 27" 4K LCD was comfortable at 150%. I'm evaluating Dopus atm, and came across the 'transition' settings so I bit the bullet and switched both monitors to 125% to take advantage of this. Now I can't live without them lol.
Therefore please, please put in the feature request for a separate 'icon' column like the thumbnail column that can be resized as desired.
Here is a screenshot of what they currently look like :
The image icons are the FastStone Viewer suite and Should show the different extensions but are totally illegible at this size.
I also strongly support this request. I hope the file icon column can also be customized independently. The default icon size sometimes looks quite bad. For example, I'm currently using a 27.6-inch 2880 * 2560 monitor, and the Windows DPI scaling is set to 150% (under this setting, the display size of most applications is acceptable). In detail view, the file icon size is 16 * 150%=24 pixels. However, many file types have icons with 16 or 32 pixels, but not 24 pixels. The rendering of these file icons at 24 pixels is much worse than 32-pixel icons. I hope to set the file icon size in detail view to 32 pixels, but currently, it's not possible in DOpus, unless I set the Windows DPI scaling to 200%, which would make all other applications' displays too large.
Furthermore, this request is also necessary from other perspectives. For instance, in detail view, the font can be customized, but after increasing the font size, the icon size still stays the same, so the display of the text and icons (especially when the name and icon are in the same column) looks very strange and ugly.
Isn’t that a problem system-wide, not just in Opus?
Those icons show up everywhere. The top left corner of most programs’ windows, File Open dialogs, listviews with icons, the Taskbar “tray”, and so on.
The OS determines how big they are, not Opus, and they’re used everywhere.
Indeed, the same situation applies to system-wide especially File Explorer;
it's just that we all wish Dopus can "do better than Windows", which is probably what Dopus was originally intended for 