Similar to the Portal Text Editor island, the Carousel island can be used in many different ways. Its versatility makes it difficult to pin down just one single recommendation for Island Property settings. Carousel Islands can house any combination of three different Frame types; Dashboard Visualizations, and User Profiles, and Image Frames. The former two of these examples do not resize as the monitor resolutions scale. However, Image Frames will, if we configure it to do so in the Frame setup configuration. With this in mind, you may want to consider keeping your Carousel rotating between frames of the same type to ensure a consistent experience with the island. This is certainly not necessary, but it does make setting the island properties a little easier. 
With any of the options listed below, it is recommended that you keep your content relatively the same size. For example, Visualizations for dashboards should be set to the same height/width so that the size of the island does not drastically change between frames. 

Carousels with Dashboard Visualization Frames

Content Width - Since the size of the content is never going to resize with different monitor resolutions, there is no need to set the width to a percentage. A percentage could potentially give us scroll bars on the island. Setting the width to lock to the content width will ensure this doesn't happen. 
Content Height / PX Height - There is not much difference in these two settings for Dashboard Visualization Frames. When configuring the Visualization Frame, we can set the pixel height of the visualization. Choosing Content Height will set the island to the height of the value entered into that field. You could choose to set a PX Height of the island if you wish. Just be sure to make it equal to or greater than the height value in the Visualization Frame settings to avoid scroll bars. 

Carousels with User Profile Frames

Content Width - Since the size of the content is never going to resize with different monitor resolutions, there is no need to set the width to a percentage. A percentage could potentially give us scroll bars on the island. Setting the width to lock to the content width will ensure this doesn't happen. 
Content Height - Choosing Content Height for Carousels with User Profile Frames ensures that the island will be tall enough to accommodate the entire list of profile fields next to the user's photo. Setting the height to a fixed pixel could potentially introduce scroll bars on the island, if the number isn't large enough. Since this island will not resize, you may wish to consider leaving extra space below this island so that it does not overlap anything below it on smaller resolutions. 

Carousels with Image Frames

% Width - Image Frames in Carousel islands behave exactly like the Image Island. This introduces a little complexity into the Island Preferences. A percentage-width is recommended to help scale as monitor resolutions change through the organization. This recommendation is paired with the recommendation of setting the Image Frame's behavior to "Stretch".
Content Height / PX Height - With the above recommendation of "Stretch" for the Image Frame's behavior, setting an exact pixel height will be necessary. You have the option of setting the desired pixel height of the image first before uploading it into the Frame and then selecting Content Height. Alternatively, you could use to set the island properties to a Pixel Height and set that number without needing to edit the photo first.  

For more information on using the Image Island effectively (which functions just like the Image Frame in Carousels), please check out the Video: Page Design with Layered Image Islands