I don't know what version of Ps you are using, for some time now Adobe has offered additional options, such as specifying the length of the longest edge. Setting both width and height to the same value has the same effect as using long edge so it doesn't really matter. When it comes to image file size, especially with JPEG, then that will change with the content of the image. Images with lots of areas with little fine detail will compress far more easily than those with lots and lots of small fine detail.
In general when dealing with Adobe products it doesn't matter if the JPEG compression scale is the 0-100 or the 0-12 one, they all only have 13 different compression levels. Because JPEG converts the image data from the usual RGB colour channels to a system based on luminance and chrominance channels similar to that of old analogue TV signals there is always a small conversion loss involved in storing an image file as a JPEG. After doing some tests I discovered that it was almost not possible to even measure the difference in the RGB channels for any pixel in an image that was saved at a compression level of 10 (80) or above. The comparison was made between an 8 bit uncompressed TIFF file made directly from a RAW and first generation JPEG versions made from the same image. There is though a much bigger difference between the level 10 JPEG and the level 12, but that is not what you should be measuring.
In general you should see a reduction of between 40% and 60% in file size between a L10 JPEG and the uncompressed image data. For a 3:2 ratio image, which is 3.22 MB when uncompressed, when saved at level 10 I would expect it to have a file size between 1.9 MB and 1.2 MB. Level 12 is basically convert to JPEG and save without any compression, and I have actually seen some maximum quality JPEG files that have been larger than the uncompressed TIFF file. File sizes will change if you crop to a different aspect ratio, for example if you make the image 1:1 by cropping ti square then it will end up being 1300×1300 pixels, and an uncompressed size of 4.83 MB. So you may also see quite a bit of change in file size if you crop to different aspect ratios too.