I need to resize my files in bulk when I export to exactly 1mb each. So far I have selected "image sizing", set at 1.0 mb 300 resolution, and the file exports at 716 kb. I also have the file size limited to 1000k in "file settings". How do I get exactly 1mb?
Not possible to specify that each file will be exactly 1 mb (and this has nothing to do with Lightroom Classic). Nor does it make any sense to do so. Probably the 716kb is the best you can do for that photo.
It isn't possible to specify a specific resolution to a digital image. The only resolution a digital image has is the pixel dimensions of the image. If the image is, for instance, 6000 x 4000 pixels then that is its resolution. It doesn't matter what the PPI setting is. If the image is smaller, like 1800 x 1200 pixels, then that is its resolution.
In Lightroom's Export dialog it is not possible to specify a digital volume (MB). Are you confusing Megabytes with Megapixels?
The file size limit in File Settings is applicable only when the output format is JPG and determines the minimum compression applied but does not affect image resolution or decompressed file size.
If you really want to approximate 1 MB, it may be useful to know that in an uncompressed 8 bit TIF one pixel is 3 bytes. Since 1 MB is 1,048,576 bytes, dividing by 3 makes for 349,525 pixels in area. Make the pixel dimensions somewhat smaller in order to leave room for the file volume of the matadata.