Central Admin Central Admin > Application Management Site Collections > Specify quota templates
Edit an existing template and choose Personal Site. Change the values under Storage Limit Values to the values you want.
Where could I find the settings for a special user? Central Admin > Application Management Site Collections > Configure Quotas and Locks
Within section Site Collections choose a users My Site.
What to do next to activate this settings for already existing users
Central Administratrion > Application Management Site Collections > Quotas and Locks
Select the site collection of a specific user you want to update, change the template to “Individual Quota”. The old values are shown within the input boxes. Change them to the new ones or just save the “individual template” and after that change it back to your template you want to use.
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