What do you mean by fixing the transform? In the transform node, some properties can be set to use relative value.
For example, the output dimension can be set to use the same size as the input, or a multiple of input. The position value can also be set to use the fractional unit, which allows you to set the position based on a fraction of the output image ([0.5, 0.5] will put the image in the middle if you set the image anchor in the middle [0.5, 0.5] as well).
You can see the Picture,When make the big project..the "blend" everytime need set pic size is very trouble ...~!