Scan photo prints to iPhoto

Question

I have a lot of photo prints taken on old-fashioned film, before the days of digital.
I have an HP Photosmart 5520 scanner/printer which has an option for Photo Preset. The default 'Resolution' setting is 200ppi. There are other options for 75,100,300,600,1200 or 2400 ppi.
My first question is, which 'Resolution' setting should I choose ?
The scanned size at 200 ppi for one print is 2.74MB. When sent to iPhoto its size is reduced to 1MB.
Another apparent anomaly is that although the prints are all 6 inches x 4 inches, their 'info' in iPhoto can vary from anything between 1184 x 791 to 2292 x 1498, and in size from 322 KB to 985 KB.
The digital photos I already have in iPhoto, nevertheless, are all 3888 x 2592 ; but the sizes vary eg from 3.4 MB to 6.1 MB.
File > 'Get Info' for the scanned photos says they are JPEG images and 'color space' is RGB, which I understand is the requisite . . . Can  the foregoing re scanned and digital photos be explained ?
Thanks in anticipation

Answer 1

My first question is, which 'Resolution' setting should I choose ?
Depends entirely what you're going to be doing with the Photos. Jpeg is a lossy format, so as you edit each time you save some data is lost. If they require intensive editing, then the more data the better. Are you going to print them? Depending on your devices and the quality you hope to achieve, then perhaps the old 300 dpi for printing is a reasonable rule-of-thumb. If you're uploading them to the internet, say flickr or whatever, then 100 is fine. So, it depends on your proposed usage.
The scanned size at 200 ppi for one print is 2.74MB. When sent to iPhoto its size is reduced to 1MB
iPhoto doesn't reduce the size of anything on import. Where are you seeing these sizes mentioned?
Another apparent anomaly is that although the prints are all 6 inches x 4 inches, their 'info' in iPhoto can vary from anything between 1184 x 791 to 2292 x 1498, and in size from 322 KB to 985 KB.
No anomaly here. Your computer has no "inches" everything is measured in pixels. So the fact that an image is 6x4 in the real world is of no relevance. What matters is the pixel dimensions you choose to digitise (or scan) it. You can create a scan of you 6 x 4  that will be 600 x 400 pixels. (@100dpi) Or you can as easily create one the will be 14,400 x 9,600 (@2,400 dpi)... In other words, don't expect a direct correlation between what you hold in your hand and what you make on the screen. They're different worlds...
But again, iPhoto doesn't change anything on import, so I'd be looking at the scanner for explanations of these variations.
As to the file size, remember not all pixels are the same. An image that is pure white or pure black will have a smaller file size than one of a richly coloured sunset, and will also depend onn the resolution you scan at - so again, no direct correlation, it's a complex equation.