EyeonTips:Script/Reference/Applications/Fuse/Classes/Image/Blur

From VFXPedia

As described above, a size of 1.0 creates a blur equal to the width of the image. The Blur tool in Fusion, however, uses a different scale. Here, a value of 1.0 equals a one pixel wide blur on a PAL image (720 pixels wide). Due to Fusion's resolution independence this will be a 2.6 pixel blur on an HD image.

In order to give the user a blur slider that works like the standard blur tool, divide size by 720 before calling the Blur method (see the FuseBlur example). To achieve a one pixel blur regardless of resolution, divide size by the input image's width.