Further to the above, I've been looking on the web for recommendations only to find that commercially influenced recommendations are common.
So, I'm confused . Can the mods please recommend a size in PPI which they think is appropriate please.
It's not the size that needs reducing but the resolution - the maximum they need to be in reality is no more than 1200 x 800 pixels, on a computer screen this is more than sufficient. The file format will then determine the actually size.
Depending up on which photo editing suite you use it may have a 'Batch Processing' option.
I use Affinity which allows me to batch process photos including size and resolution. I generally reduce the photo size to 1000 or 1500 pixels on the long edge and file size to around 300kb for fast uploading, especially on mobile devices.