Windows users can use IrfanView. When saving an image, the savings dialog allows you to specify a particular file size OR a quality setting (controls the degree of jpg compression).

A setting of 96 or 97 is probably much more quality than is absolutely necessary, but will work to decrease the file size slightly. Decreasing pixel count (image dimensions) is recommended by some people but not by me. Fewer pixels WILL result in a smaller file size but the decrease in image quality is probably greater when decreasing pixel count than it would be by increasing jpg compression (my opinion here -- have not studied this [yet]). It really depends on the end use of the image. If it's just going on the web, use Paul's recommendation since most monitors are limited in pixel count. If the image is going to be used in an application where resolution is important, the full size image will be better.

A couple years ago I studied the degradation of jpg images with multiple re-compressions, and in the process measured the RMS change in an image from an original tif to a jpg as a function of quality setting. The abstract is at https://www.uglyhedgehog.com/user-page?upnum=3000 and that page contains a link to the full study (14 pages). I did not see any noticeable degradation of images with quality settings of 70 or more. Of course, some people are more picky than I, and some images will probably have larger changes than others, so this is a subjective determination.

All the programs I used to write jpgs had a quality setting range of 1-100 except Photoshop, which has a quality range of 0-12. It appears to me that the comparison between Photoshop and the other programs is not linear, but I have not tried to measure it. My estimate is that the Photoshop quality setting of 0 appears to be equivalent to something in the 30-50 range of the other programs.

I submit pictures to a website that limits pictures to 12 mb size or less. The files I get are often 14-18mb in size so I need to reduce them. Since all my pictures are jpeg, I usually reduce them by setting my jpeg compression to 97 or 96 in my editing software. There is also a resize option where I choose pixel size reduction. I don't know if these two options work similarly or differently. What would be the best method to reduce pictures size from 14-18 mb to under 12 mb? [I use an old version of a semi-obscure editing program called Zoner Photo Studio but this question would apply to Adobe Elements too, I think.]

Are there any programs that allow you to save by file size, so I could just pick 11mb or 10 mb for example? When I save by my current method of changing jpeg compression, I really don't know the resultiing file size until after I do the save. If I am just missing something obvious, please gently let me know.

Since file reduction is a math thing on the computer, it could be done by file size, BUT I have not heard of it.
Despite this submission rule, it is not a very useful feature. Picture size could vary quite a bit.
I would go totally by size reduction and maximize the quality setting. Going thru a few folders of pictures should set you up with solid expectations.
If you are willing to settle for a 10mb aim point I would expect all of your pictures to be acceptable. A little observation and remembering the results make repeat operations a snap.
10mb is overkill for viewing, and makes a good small print.
Boris

Since file reduction is a math thing on the computer, it could be done by file size, BUT I have not heard of it.
Despite this submission rule, it is not a very useful feature. Picture size could vary quite a bit.
I would go totally by size reduction and maximize the quality setting. Going thru a few folders of pictures should set you up with solid expectations.
If you are willing to settle for a 10mb aim point I would expect all of your pictures to be acceptable. A little observation and remembering the results make repeat operations a snap.
10mb is overkill for viewing, and makes a good small print.
Boris