The default mailbox size would be set in User Defaults whereas the default size limit for the domain would be set under Domain Defaults. Both of these are found in the web interface on the left side menu when logged in as a System Administrator. 

Hey Kyle, so i can set this as a default value so whenever i create a account its set at that storage size?

Correct, 

One item to be aware of is that there are two (2) places to set this User Default setting. 

These are the default user settings when creating a New domain. This is where you would adjust the setting to be applied to any New Domain you create. 

This is where the settings can be set up and customized per domain after they have been created. So if you wish to adjust the setting for a Current domain, this is where you want to adjust it. 

Hope this helps. 

~Tony

Thanks