In the OWL Cinecam and Capture Components the Filmback aspect ratio settings automatically update your Render Target resolution:

1. The resolution settings for your Render Target are in the OWL Capture section of the details panel. You can customise the 'y' resolution as you need (1080 for HD, 1440 for 2K, 2160 for 4K etc.) 
2. These are responsive to the aspect ratio of your Filmback settings. For example, 16:9 ratio is 1920:1080 resolution:
3. If you update the aspect ratio in your Filmback settings, the 'x' value of the resolution will automatically change as long as 'Constrain Aspect Ratio' is ticked in your 'Camera Options' section (see update for Super 8 below):
4. If you untick 'Constrain Aspect Ratio', the tickbox next to 'Resolution X' will become unlocked and you can customise your 'x' resolution. This is not advised unless specifically desired as it may distort your image: