Hello,

I'm wondering how the custom sizes attribute works for single images. For example:

In the attached screenshot, I want an image to display at 300px wide for screens 760px and below. 

I tried inputting what i thought would work, but im not seeing any change to my image. 

Am I formatting it correctly? is there something im missing? More details on how to use the custom "sizes" field would be very helpful. Thanks!