Can someone tell me why this photo might have been rejected? Included camera settings.

Hello,

From your camera info, you used an f-stop of 5.3 and 72mm focal length. F 5.3 gives you a shallow depth of field in conjunction with the focal length. This has resulted in the blossom not being fully sharp - the depth of field is too shallow.

It would be a good idea to take several shots and increase the f- stop.

But then take note of your shutter speed. 

Understanding shallow depth of field photography | Adobe

Nice picture, but it is lacking focus. Check that 100%. You could also enhance the contrast.

The blurred background is OK, that even adds a nice texture.

Why would the file not be big enough? 24Mp is much more than the required 4MP, so you are well equipped.

It's not all about size.  It's about your subject, composition, lighting, focus, color balance and all the other things that go into good quality images.

Stock already has 17.5 million pink flowers.  Even if this is accepted, it's unlikely to get viewed, much less sold.

https://stock.adobe.com/search?k=pink+flowers

If you want to be a Stock a Contributor, do some homework first. 

• Read the entire Contributor User Guide.
  https://helpx.adobe.com/stock/contributor/help/submission-guidelines.html
• Find subjects that Stock needs. A creative twist on mundane objects can be good sellers.
• Plan and execute your photo shoot with various camera settings, focal distances, lighting, etc... 
• Choose your highest quality images and make needed corrections in Photoshop or Lightroom.
• Compare your best work with current Stock inventory.
  • Ask yourself if it's as good or better than what Stock has.
  • As a customer, would you buy it?
  • What would you use it for commercially?

Hope that helps.

Casual shots of flowers are almost always rejected by Adobe Stock. The database is already filled with millions of flower images, so submissions must be extremely unique to get accepted.