Lightroom export 20 mb feature

I had a look at the Showit website, and found no mention of a 20 MB file size requirement.

What they do recommend is 3500 pixels on the long side.

If this is for jpg, file size is not an indicator of image quality. The file size of a jpg is largely dependent on image content.

An image with lots of sharp, busy detail will have a much larger file size than an image with predominantly smooth, flat or out of focus areas. If you have an image in the latter category, it may not be possible to reach 20 MB, even at 100 quality.

However, the feature you're requesting does already exist in the Export dialog, Limit file size to.

The problem with using this feature is that you have no control over the Quality setting, and may end with an image with visible compression artifacts and unacceptable quality.