There are two ways you can set the preferred resolution served in your videos. You can either set a guideline in your Theme or you can specify the exact resolution set for your embeds in your Publishing options.
Resolution Set via Theme
Browse to your Theme >> Theme Advanced Options >> Publishing.
The "Streaming Optimized" will generally serve the resolution in the middle of what is available. For example, if you have 360, 720 and 1080 available then 720 will typically be serviced. If you have 360, 720, 1080 and 1440(2k) available then 1080 will typically be served.
Resolution Set via Publishing Options
When you publish your video, activate this option and choose the resolution you want to serve from the drop-down. The drop-down will display all available resolutions for that particular video:
**Note** If you have configured both options for a video, then the option configured in the individual published video will over-ride the theme setting.