TC-780A memory upgrade

I want to add memory. My TC-780A came loaded with one 8gb stick. I have two slots and 32gb capability. I want to add a 16gb stick.

I would like to know the best place to buy the stick and would like to know if it matters which slot has the larger stick. Do I need to put the 16gb stick where the 8gb stick currently is and move the 8gb stick to the open slot?

Thanks. John from St. Petersburg, Florida