Increasing size of legend symbols in QGIS3.12.2 print layout

I have pixels with a fixed size on the map, as they have a certain resolution. Now in the legend of the print layout the symbols have the same size as the pixels on the map.

I want to increase the size of the pixels in the layout to create a legend, see the image below how it looks like now:

And what I want, and what I have with other images is a legend that looks like this:

How do I go about doing this? I tried to increase the size of the pixels using the item properties, but none of these actually increase the size of the symbol.