How to get larger thumbnails in dolphin

With Dolphin:

Not a perfect solution but you can change some Qt environment variables only at run time so it doesn't affect your global settings.

Namely:

• QT_SCALE_FACTOR (float): scales everything up or down. More info in this Qt6 doc article
• QT_FONT_DPI (int): scales text only. Used here to compensate for the global scaling. For values below the default of 96 to have any effect, you need to also set the next one:
• QT_AUTO_SCREEN_SCALE_FACTOR=0 More info in this Qt5 doc article (perhaps no longer available in Qt6?)

For example, run:

bash -c "QT_AUTO_SCREEN_SCALE_FACTOR=0 QT_SCALE_FACTOR=3 QT_FONT_DPI=60 dolphin"

(the bash -c is only useful if you make a launcher out of it)

Note: Icons and spacing between elements are not compensated for, unlike text. This is why it's not a perfect solution.

If not, is there any other file manager which supports larger thumbnails

Yes, there are 2 :

Thunar (from XFCE):

Since XFCE 4.18, Thunar let's you zoom to get up to 1024px wide thumbnails, without changing any preference. Just hit Ctrl + + or scroll with Ctrl + MMB while in any View mode, be it Icon View , List View or Compact View and there you go. However, it currently has 2 noteworthy issues:

• https://gitlab.xfce.org/xfce/thunar/-/issues/1143
• https://gitlab.xfce.org/xfce/thunar/-/issues/117

Caja (from MATE):

For Caja on the other hand, while you can go even beyond 1024px (there's no limit), you need to at least change a gsetting, either with the command shown below or with dconf-editor if you prefer to use a GUI.

However, the gsetting solution is limited to thumbnails of images, videos, pdfs, etc... but doesn't apply to vector icons such as folders and file types that don't generate thumbnails. If you want a bigger size for everything, you have to edit the source code like explained in my second solution.

1st solution (simplest but limited) :

The default 100% thumbnail size is 64px. In this example I change it to 200 but you can put any value. Although, note that by zooming in Caja to 400%, this size will be multiplied by 4. So in the example below, the value of 200 allows you to get 800px thumbnails by zooming with Ctrl +.

gsettings set org.mate.caja.icon-view thumbnail-size 200

Or with dconf editor just go to org/mate/caja/icon-view/thumbnail-size, disable "Use default value" and enter your own value.

You may want to enable "Use compact layout" in the preferences so your folders don't get unnecessarily spaced out because of the large thumbnail size.

Note: the displayed file names have a hard-coded max width per zoom level, meaning if you have very large thumbnails and long file names, the names will not make use of all the thumbnail's width and add unnecessary line breaks. So you may also want to change the ellipsis limit from 3 to 1. This way, instead of long names split in 3 lines you'll get only one short line with a "..." at the end. Not ideal but at least more compact and less ugly:

gsettings set org.mate.caja.icon-view text-ellipsis-limit "['1']"

You can also opt out of displaying additional information about the file when zooming closer by going in Caja's preferences in the Display tab and setting the 3 dropdown menus to None.

Here's the result on a 4K screen:

2nd solution (source code, more complete):

Note: if you use this solution, then you should not apply the first one (you should keep the gsetting default thumbnail size to 64px).

1. uninstall caja
2. clone the github repo
3. in caja/libcaja-private/caja-icon-info.h, you can change the values in the following lines (preferably multiples of 192, like 384 and 576)

#define CAJA_ICON_SIZE_LARGE    72
#define CAJA_ICON_SIZE_LARGER   96
#define CAJA_ICON_SIZE_LARGEST     192

However, you shouldn't change the value for CAJA_ICON_SIZE_STANDARD, as it will break the zoom such that you won't be able to zoom to large sizes.

4. I also recommend decreasing the icon padding (margins around each icon), otherwise they will be absurdly spaced out. For that, go in caja/libcaja-private/caja-icon-container.c and change the values in the following lines :

#define ICON_PAD_LEFT 4
#define ICON_PAD_RIGHT 4

#define ICON_PAD_TOP 4
#define ICON_PAD_BOTTOM 4

#define CONTAINER_PAD_LEFT 4
#define CONTAINER_PAD_RIGHT 4
#define CONTAINER_PAD_TOP 4
#define CONTAINER_PAD_BOTTOM 4

#define DESKTOP_PAD_HORIZONTAL  10
#define DESKTOP_PAD_VERTICAL    10

icon_width = (bounds.x1 - bounds.x0) + ICON_PAD_RIGHT + 8; /* 8 pixels extra for fancy selection box */

I personally set them all to 1 (for the last one it's the + 8 that becomes + 1)

5. Follow the build instructions on the github page until sudo make install

6. Here's the result:

7. Coupled with this neat trick, you can also get folders to show a thumbnail of the first image inside them instead of a folder icon. Here is the same folder as above, using this trick:

Final note:

You might find the same lines in Nautilus's or Nemo's source code and you may even find a way do do it for Dolphin. However, I recommend keeping at least one file manager "sane" for everyday use, as these modifications take a toll on performance. I've had Caja crash and freeze several times when it had hundreds of thumbnails to generate.