Increase resolution when using the "create collage" function

I don't use the collage feature myself because I have never needed it. As indicated by @hatstead It's so easy to customize your own collages with the normal tools of Elements!

However, I have been curious to see how you could print the collage with 300 ppi on an A4 paper for instance.

No problem in the print dialog to choose the paper size AND the print size on that paper, for instance 8" x 10".

I have checked in two ways that doing this does indeed keep the 300 ppi for the new 8" x 10" print size.

First, since I did not find a way to switch to the 'Expert' mode, I just went to the location of the project (the .pse): it showed the layered full size .psd version. The layer palette shows that the collage is built on "4 x 6" canvas  on which all elements are smart objects. That means that if you resize that file to A4 or even A3, the collage will use the original photos at their full resolution to place them in the new canvas. Tick the 'resample' box in the dialog box: resampling the empty canvas won't change anything and that will only upsample your image layers if their resolution is not enough (not your case). The A3 output is tack sharp.

Another way to check is to use the print command to a virtual pdf printer like the 'Microsoft print to pdf' included in Windows 10. You then open that pdf in the editor while specifying the size and resolution. Same result as above.

If the above seems too technical, just concentrate on locating the .pse project on your computer. Then open the .psd layered file in the editor. Open the menu Image >> Resize >> Image size. Enter your new dimensions and resolution while having all 3 option boxes checked. Save in your prefered format, jpeg, psd, tiff...

If you do it manually you will have complete control. You reference 300 DPI (dots per inch) which is determined by the printer. Resolution is expressed in PPI (pixels per inch). 300 px/in is a generally accepted resolution value for high quality prints.

Open your picture files, and go to Image>Resize>Image size and enter the values appropriate for your project, esp. 300px/in. Be sure that "constrain proportions" is checked. This will resample the file. Save it (save as) with another name so as not to overwrite the original.

After this is completed, go to File>New>blank file and enter the dimensions for your canvas, resolution=300px/in. ok it.

Copy/paste the 3 files on the canvas.

We look forward to your progress.