Contact your seed supplier. A quality vendor should have reliable advice about which plate to choose for the seed size you purchase. Be prepared to inform the dealer of the brand and type of planter you use, as well as your planter's available sprocket settings.
Find out the planter plate size from the seed brand packaging. The seed company may publish a planter plate size chart on seed packaging or on its website. If not, phone the seed company's customer service to get planter plate information for the seed size you purchased.
Get in touch with your plate supplier. Though vacuum-driven planters are more and more prevalent, vendors who supply plates for gravity-fed planters are knowledgeable about size selection issues. The supplier may have an informative website, as well as helpful telephone support. Take advantage of both if necessary.
Contact your state or county extension office or university agriculture department. Many extension offices are dedicated to helping not only home gardeners but micro-farmers as well. A corn planting expert may be on hand. If not, the office should be able to direct you to a knowledgeable person or organization. A nearby university agriculture department that is familiar with corn planting issues in your region is another quality information source.