What size cover do I need to buy

The product support page for the Chromebook allows you to find the system configuration for your system by entering your service tag or express service code.

Once you enter this information, a list of parts will populate that show you the individual components of your model and the part number

Your question did not specify if you were referring to the battery cover, display cover or other cover, but all the individual components on the system configuration will be listed with the part number.

Once you find the part you need, I recommend you use the Contact Information to see about the availability of the part you need by contacting sales.

Alternatively, you can always you online retailers like Amazon or eBay to search for your part by name.

I did review the manuals page and there were no specific model service guides, but they did provide a battery replacement sheet if you are replacing the bottom cover.

If you were referring to a cover for the LCD screen or other visual/cosmetic covering, I can find some suggestions for your model as well.