Fluid size for native ads
Fluid is a native ad size that allows more flexibility when styling your ad. Ad Manager automatically sizes the ad by filling the width of the enclosing column and adjusting the height as appropriate (just like a regular HTML div on your site). This size works in combination with the fluid slot type in Google Publisher Tags or the Google Mobile Ads SDK (Android / iOS).
To use the Fluid size, first choose a method to create your native ad and then choose Fluid as the “Ad size.”