In order to maximise the resolution you need to make sure that you start with as large an image as possible. For the largest size you mentioned – 11×14, to print at high quality (300DPI) you’ll need to start with an image that’s at least 3300×4200. To do this, you will need to first change your Image Quality setting to Archival (you can find your settings by clicking Hi Username! on the top right toolbar within the editor).
Let us know if you have any other questions!