P: Add File Size in MP as an Export Parameter

When exporting a copy of a file from Lightroom (cloud-based, not classic) I'm only able to adjust the output with either "long-side pixel dimension" or the very ambiguous "quality %", but nearly every time I'm exporting anything other than 100% size and quality, it is because of a file size limitation on someone's upload portal or their required submission standards, so all I'm really intersted in when exporting in this way is file size in MP.

I would LOVE to be able to dial up or down the export file size (not dimension, but MP size), and just have the corresponding pixel dimensions go up or down accordingly (with fixed proportions of course), so that if a site requires a file that is under 5MP, and I want to get as close to that limit as I can while not exceeding it, I can just move the slider until the file us under that 5MP limit while the overall image size would just move up or down as a % of the original size.

In the current setup, the only option is a clumsy game of guess-and-check, slowly trying to narrow in on the long-side pixel dimension that will yield the desired file size, which takes a lot of time and effort even for 1 image, let alone many.