I scanning in my old paper photos from the 80s and 90s.
What DPI level should I be scanning them in? I've tried 150dpi (default) and 300dpi and don't see any difference (other than the size of the resulting image).
Scan them at a high enough resolution that you'll never need to scan them again. i.e. "archival" quality. I'm assuming the old photos are important and you want to keep them for a long time, and view them both on screen and perhaps re-print later.
Generally, space is cheap, but your time isn't. I suggest 600dpi minimum, and more if you can handle it.
Finally, make sure you save your scans in a non-lossy format such as PNG. Avoid JPG as an archival format.
If you ever want to put the images on a web site or email them, you can scale them down and use a lossy format at that point.
While the other answers, such as scanning at 600dpi, are generally true, if your scanner is of a low quality, you might not be actually picking up any more detail.
Compare the different sizes. Zoom in, look at the details, try a few prints. Show them to a photographer friend if need be. If there isn't a worthwhile difference, don't worry about it. 150dpi seems a bit low, but depending on the output you may not be able to notice a difference above 200dpi.
Also, if you do save the files as JPEG, just put the quality up at the highest or next to highest. You always hear don't resave JPEGs, but on highest quality setting, the amount of data actually lost, and hence artifacts, is negligible. The "don't resave JPEGs" comes from saving web-optimized graphics multiple times, which does look hideous. (That said, there isn't really any reason not to use PNG, and I would recommend it.)
If you are certain you can't see any perceptible difference then I'd say keep it at 150dpi. It sounds a bit low though, are you looking at them fitted to your screen?
If you only ever intend to view them on-screen then 150dpi is probably well enough. If you intend to print them, make sure the difference isn't perceptible there too! I would expect you'd need more like 300dpi for printing, unless the originals are of a less-than-perfect quality.
Lastly, if you're worried about file size play around with the JPEG compression setting. You may then be able to scan them at a higher resolution and still get smaller files with imperceptible quality loss.
Larger dimension of the same image should give you higher resolution and detail when you view/print to the same size. Unless it is digitally enlarged or due to a poor scanner, higher DPI is preferred. But you have to balance off with the size and also extra 'noise' introduced.
I'd say anything 600 dpi up is pretty good for photos, nothing lower as when you would want to edit them the colors and resolution would not be good enough
Most people have suggested no less than 600 dpi I have been using 1200 and even then sometimes the original photo is so low resolution can't get larger copy of it.
Dpi settings are for dots per inch, and most photos are printed at 300 dots per inch, when you scan your original photo at a higher setting it doubles the resolution and size you can print that photo. For example.
4x6 Photo scanned at... Can be printed at... 600 dpi. 8x12 1200 dpi 16x36
I hope this clears up any miscalculations of dots per inch.