In the file export dialog, under File Settings you can set limit file size to, when creating jpgs. That is because jpgs are a compressed format and the compressed size varies according to many factors.
When exporting files as uncompressed Tiff or psd, the file size is defined by the number of pixels in the image and the bit depth the image it is exported to (and a little bit extra for the metadata). It is best to export to a size required by the end usage. For instance for web use a picture may be limited to 800 pxls on the long side, 60 % compression as a jpg, whilst for print use you would size to the exact dimensions of the paper and export at around 95%.
As you have the original file, derivative files are only required for a particular usage and therefore sized according to the usage required. Original files are untouched and remain in the quality first imported. For the best quality import RAW files.
The formula for uncompressed file size is
File Size = (pixel dimensions x bit depth) / 8
files refereed to as 8 bit are actually 24 bit (8 bits per channel) and 16 bits likewise is 48 bits (16 bits per channel) for this calculation)
Selecting a particular file size in MB's for uncompressed files would require LR to resize images in an almost random way without you setting the pixel dimensions and would be completely pointless. You can re size to any pixel dimensions you require and this defines the overall size of the file except in the case of compressed formats where the ability to limit file size already exists.
I formula that worked for me is MegaPixels x 3 = File Size.
This formula is not mathematically exact but gives a good approximation for 8-bit files.
So if I want a file size to be just over 50 MB at 8-bit I chose 17 or 18 MegaPixels in the LR Export dialog.
So, if you want a file size of under 24 MB try 7 or 8 MegaPixels.