Hi ertert,
please have a look the this thread Exporting RGB image as a Geotiff
There I explain how to create a subset if not enough memory is available.
Instead of using a smaller region you can also set the step-X/Y values in order to sub-sample the image.
Maybe an other option is to increase the memory heap size. In the ‘etc’ folder of the installation directory you’ll find a file named snap.conf. Open it in a text editor.
There is the line which starts with ‘default_options=’
In this line you’ll find an option like -J-Xmx5G. Increase the value. You could use something like -J-Xmx13G, if you have enough memory in your computer