The file size in megabytes is not meaningful.
The file size in megapixels is the correct metric to be looking at.
300 ppi is certainly good, but not necessarily needed. 220 ppi works great in many circumstances. Even 150 ppi at times.
The important part is to make sure you are not cropping out too many pixels during the development of your images.
Can you quote the exact word-for-word requirements for the JPG photos from some specification provided by Mixbook? Better yet, show us a screen capture of said specification.
Photos need to be in .jpg format. For best print quality, photos should be at least 4MB at 300 dpi. The maximum file size at this time is 15MB. Files are recommended to be in sRGB profile, but CYMK files are accepted as well. We also recommend for you to make any edits or color corrections prior to uploading your photos.
This is utter nonsense, as also pointed out by @maniacjoe. It's the megapixels that matter, not the megabytes.
Nevertheless, I would recommend you export the photos leaving "Resize to Fit" unchecked, and a quality of about 80. If any of them come out at >15MB, which would cause Mixbook to reject the photo due to their moronic specifications, then you'd have to re-export the photo with some resizing.
I'm not at my computer to open Lightroom CC. But if I recall when exporting files there were a lot of options. This was the part that was confusing. What to add or not for the export.
At the moment, the only options necessary for you to fiddle with are:
Uncheck "Resize to Fit"
File format is JPG
Color space is sRGB
See if that combination of options gives you files that meet the specifications.
And I forgot one:
set Quality to 80
Thank you all for the feedback I appreciate your taking the time.