Hello, my friend,
Well, the resolutions field is there for you to change in order to fit your needs. These three device widths that you see there are just some nice defaults based on contemporary devices.
You could totally change them as you wish, but I would recommend not adding too many of theme. Nothing wrong will happen if you do, but you will end up having tons of thumbnail images even for very close dimensions and sizes in kilobytes, which will result in very tiny and unimportant gains between each other.
Our suggestion is to have as few as possible, carefully selected resolutions (I see no real reason for something than 2 to 4). However, if you feel you need to change the default ones, make sure you understand how the resolutions work in the plugin description page and study the statistics of the most common devices which are out there today. Then try to select what is best for you.
Hope I helped!
Let me know if there is anything else I can do for you.
Cheers,
Takis