How can I update a dimension based on upload image dimension?
If you want to let customers crop a custom dimension that they can enter using a dimension input, follow the steps below.
Template
In the App / Templates page, you can use this template as a starting point for your own configuration:

Dimension option
Create an option of type Dimension to let the customer enter their desired dimension:

Upload option
Create a new option type Upload:

On the Advanced tab, Connect the Upload option with the previously configured Dimension option:

Enable the Dimension update checkbox:

Upload an image to verify that the dimension X/Y values are updated:
