I would like to change the collage format. The program has a (permanent?) 6x4 format. I tried to change it to 3x2 or set pixels to A3 or A4 format but I can't. I don't see this option. No change in the collage format is a nonsense.
It is possible to re-size although it requires some user manipulation of the layers in advanced mode. I’ve never done A3 but have scaled up to 8 x 10. You can also create your own document size and place photos manually although you don’t get the benefit of the interactive templates. See if any of my YouTube videos help.
If we use your solution to solve the original question, that is to resize the automatic collage to a bigger final dimension and a different aspect ratio, we save the result as .psd (or .tiff) and we open that .psd and resize it.
I think it's important to understand why upsizing does not lose any quality or resolution from your original assets.
Let us look at the layers panel of the .psd in expert mode. All layers are smart layers. That means they refer to the original files. You can double the size of a file and even more, the default size of the template is only there to fix the aspect ratio. Same for possible text layers which are vector layers.
The A3 print size is not generally available if your printer does not support it. So, you have to resize manually the .psd to (42 x 59,4 cm) before printing.
I have always created my collages manually (which is not so difficult), but I must say that using the automatic collage as a first step may save a lot of time. Thanks again, John!