I would like to retrieve the site space using the "Web Call Service" action and I am not sure what I need entered to get that.
Are you looking for usage data? like how many MB are used on the site via a workflow/web request? I don't think there is a method on the site object. But there is a Web Request call you can make to the OWSSVR.DLL. But it looks like you will have to parse a lot of data that is returned.
From GetUsageBlob Method
The GetUsageBlob method returns all usage data for a site, while the GetUsageData method of the Microsoft.SharePoint.SPWeb class returns only 2000 records. GetUsageBlob works for both GET and POST operations, but the returned data cannot be displayed in the browser.
I haven't made this call before, but here are some links on how to make the call. Hopefully this is useful.
GetUsageData by WebService
Exploiting the value of OWSSVR.DLL in SharePoint 3.0 - Blog
But what is your automation need for the usage data? There are several ways to review the usage data, maybe sending a link to the Storage Metrics would suffice?
I will have to review my notes on the OWSSVR.DLL. Hopefully I made notes, because I have used that one before and I can't remember what for. I was able to use the "Build string" action with it to get what wanted.
We have over 7000 site collections and the site collection owners come and go making the ability to send out site space notification unusable unless we are contacted to change it in CA or some groups will want multiple people notified when the default notification is only two people.
Retrieving data ...