MSTech Image Resize is on GOTD today, and one of the questions in the comments I think Maybe brought up a good point: Image sizing can be more complicated than it appears...
L. Cleveland Major asked: "Does this software reduce or increase the file (hard drive storage) size?"
A digital image [photo] has 2 metrics -- the image size and its dpi. It's somewhat confusing. The dpi is kind of how densely the pixels [dots] are packed together, e.g. 300 dpi is pretty much standard for printing. Opening a photo in PSP, its dimensions in inches is ~47" square at 72 dpi [screen or monitor resolution is 72 or 96 dpi], but that drops to ~11" square at 300 dpi for printing. If I wanted to print on 8 X 10 photo paper, I could increase the dpi until I got slightly less than 8" square -- I won't lose any of the original data, and the increase in dpi, while theoretically increasing the quality of the print, would most likely go unnoticed.
So you decrease the size a photo's displayed at, & the size of that printed photo by increasing its dpi, but the amount of data, the number of pixels in the photo does not change, so the file size remains the same. You can of course take away data [pixels], and that makes everything smaller, the displayed & printed sizes as well as the file size. But, just like enlarging a photo digitally, reducing one relies on some guessing on software's part, and some quality will be lost. Software that doesn't let you set the dpi, e.g. MSTech Image Resize, normally just works at 72 or 96 dpi, so reducing a photo's displayed size reduces the amount of data [pixels].
So. you don't want to send a relative a photo that you've resized permanently, by removing pixels, if they might want to print a copy, though it is perhaps good etiquette to shrink that photo if it's only ever going to be seen on their cell phone. That said, the photos you see online or in emails are very often much larger than they appear -- it's not common practice any longer to shrink photos to the exact dimensions that you want them seen on-screen. You can usually send a photo as-is without worrying that when it's viewed it'll take up the entire display, or possibly extend far beyond the display's borders -- it's more than likely any software used to view that photo will automatically size it for display purposes automatically.
As for shrinking the file size itself, most often that's what the jpeg format's for. A Photoshop file that's ~350MB = ~10MB as a jpg at 100% quality, but can shrink to less than 2MB as the quality setting is decreased. Jpeg is a lossly format -- you lose data, same as with mp3 audio files. Also like mp3 audio files, the higher the quality, the less data that's abandoned. The quality setting you use is a compromise between how small you want/need the file to be, v.s. the amount of quality you want to preserve.
L. Cleveland Major also asked: "Does this software reduce or increase the pixel size?"
There actually is no such thing as a pixel size -- it's simply defined as the smallest thing or shape or dot that a display can show. While most people just go by the screen resolution of a display as a relative measure of its quality, the pixel density reflects how small those dots are -- the pixels on a 1080p display that's 50" across are MUCH larger than the pixels on a cell phone screen that's also 1080p.