What I would like to do is grant permission for a domain account to log on to any server/workstation and be a local administrator with having to add this account to domain admin group?
This account only needs to be able to read folder sizes on all folders on a workstation/server.
Is there a GPO for that?
You can use Group Policy Preferences to update whatever local group to contain whatever users you want it to, including the local administrators group.
Yes, this is definitely possible with a GPO.
You need to be careful though, that the GPO that makes the user a local admin on every machine does not also apply to the domain controllers, because a local admin on a DC is a domain admin.
It's all just a matter of your particular OU structure, where the computer accounts are, if/how you configure "Enforced" or "Block Inheritance," and/or WMI filters. There are too many different ways to accomplish it to really go over them all.
For instance, a common scenario is to do something like apply a GPO to the "Accounting" OU that makes all members of the "Accounting Dept Admins" local administrators of all the computer accounts that reside in the Accounting OU.
great how-to here: http://www.windowsecurity.com/articles/Using-Restricted-Groups.html