Well, I thought you wanted to use manual sliders for scaling the bbox.
If you want to pass in a list of numbers, you will need a document regeneration mechanism, which means Revit has to refresh its database before exporting the images.
You may find doc regenerate nodes, but if you could not, right after the watch node, before exporting the images, add a python node and write the below code
I tested this with this sample result, so each image is different.
