Open a base layer and open images as layers, then scale layers - was shown in the video. (0 views, that was a waste of time 
)
As before, scale up or down by an appreciable amount will result in some degradation of the image.
If the individual images are very large then process each individually.
Do a little planning and determine the image size required for your background. I still recommend one of the Gimp presets, These are already set to 300 ppi for printing if you need to print, anything more than 300 ppi is overkill. Open one up and set a few guides up to determine sizes in pixels
Scaling down each image can be improved by a small pre-blur. Any borders etc, can be cropped at this stage. A final sharpen can be used. Then save these as .xcf files.
Open up your blank background image, Open-As-layers to import those .xcf files. Keep adjustments at this stage to moving the layers, adding decorations, text..
When finished save as an Gimp xcf file. It will be large, going back to the original question on file size, that A4 2480 x 3508 pix equals
gimp xcf 16 MB compressed xcfgz 12.5 MB
export as png (lossless) 9 MB
export as jpg (85 quality lossy) 1.3 MB
Just my view on the subject, lots of other ways.
)As before, scale up or down by an appreciable amount will result in some degradation of the image.
If the individual images are very large then process each individually.
Do a little planning and determine the image size required for your background. I still recommend one of the Gimp presets, These are already set to 300 ppi for printing if you need to print, anything more than 300 ppi is overkill. Open one up and set a few guides up to determine sizes in pixels
Scaling down each image can be improved by a small pre-blur. Any borders etc, can be cropped at this stage. A final sharpen can be used. Then save these as .xcf files.
Open up your blank background image, Open-As-layers to import those .xcf files. Keep adjustments at this stage to moving the layers, adding decorations, text..
When finished save as an Gimp xcf file. It will be large, going back to the original question on file size, that A4 2480 x 3508 pix equals
gimp xcf 16 MB compressed xcfgz 12.5 MB
export as png (lossless) 9 MB
export as jpg (85 quality lossy) 1.3 MB
Just my view on the subject, lots of other ways.