Thanks for taking the time to participate here on GoDaddy Community! I updated your post title and moved your question to the 'Building and Managing a Website' board as it seems like a better place to get the help you're looking for.
I'm not sure the API key would directly affect the way your size chart displays. It seems like that would be related to the theme you're using. However, someone else in the community may know exactly what you have to do. If not, you may also consider reaching out to the plugin's developers on their website.
JesseW - GoDaddy | Community Manager | 24/7 support available at x.co/247support | Remember to choose a solution and give kudos.