I was looking for this too. I wanted to ensure that image size would automatically be converted to a maximum height or width of (say) 768. My workaround is this.
The "Resize" node, when set to proportional, only needs the width argument. It'll calculate the proportional value for height itself. So you can simply enter a value of "1" for height. When the height is the larger of the two, rotate the image to make it the width then rotate it back. You can then set the Resize width argument to a target number (e.g., 768).
This way the float operations are handled internally by Resize.