Corn seed sizing has always been an important part of the seed ordering process. 
For generations we have felt that a medium flat would be the right choice for finger pickup planters, or possibly a round size would be better to meter on a vacuum unit.  
Other considerations included the fact that the germination rate is dramatically affected by seed size.  Research has proven this is not an issue when seed is planted into good conditions. When moisture is the limiting factor, germination can improve only slightly with considerably smaller seeds.    
By contrast, it is a requirement that all seed be tested to meet high standards of warm and cold germination before leaving our facilities.    
The reality is that in seed production there are many factors that can affect what sizes are produced.  Primarily, different sizes come from different parts of the ear. The smallest are at the tip, mediums through the middle and large seed comes from the butt end of the ear. 
Production environment is also a large factor. Was it too hot and dry at pollination, or were conditions optimal? Just like in hybrid production, kernel rows are decided early on by the plant and these differences can quickly decide how large the final product is.  
As well, some genetics will typically produce large seeds or small seeds, heritable from the female parent.  
How do we manage with different sizing?  
At PRIDE Seeds we have developed the TrueFlex grade system which uses seed grade weights to help optimize plantability. Each grade is run through various meters to develop recommendations, and we use third party testing to verify maximum planter performance and settings.  
Regardless of your paint colour, many seed sizes can be accommodated with some fine adjustments to the planter!
As seed gets into warehouses, sheds, and then planters, remember that we want to optimize planter performance and understand seed sizing adjustments necessary for a picket fence stand.   

Resources:

http://agrigold.com/universal/articles/corn-seed-size-considerations/

https://www.agry.purdue.edu/ext/corn/news/articles.98/p&c9807.html

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1045&context=abe_eng_conf