It seems you are following this topic about Host storage.
I'm not sure why the Azure portal is not allowing you to alter the settings on this page. It's the right dialog...
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You can check if the changes are picked up and being sent to the device on the last page of the 'Set Modules' wizard. There, the deployment manifest content is shown.
For more information about the use of host storage, check out my blog about IoT edge offline capabilities.