Reducing mailbox size Outlook 2013 and Outlook 2016

Reducing the size of a mailbox can be achieved in three simple steps as set out below:

1) Create a Personal Folders (PST) file.
2) Create a subfolder in the new Personal Folders.
3) Move a message to the subfolder.

Create a Personal Folder (PST) File

1. In the Home tab, click New Items > More Items and choose Outlook Data File.
2. In the New Outlook Data File window, type a name for your Personal Data File, choose a save location and click OK.
3. You can add an optional password at this step. Leave the password fields blank if you don't want one. Click OK.
4. The data file will appear in the folder list in Outlook's Mail view.

Note: If you already have a Personal Folder file you wish to use rather than create a new one, open it by going to the Home tab, clicking on Open and clicking Open Personal Data File.

Create a sub-folder in the Personal Folders

1. Right-click the new Personal Folders you just created, and click New Folder.
2. In the Create New Folder dialog box, type the name for your sub-folder.
3. Under Select where to place this folder, verify that your new personal folder is selected, then click OK.

Move a message to Personal Folders
Move by dragging:
Select the message and drag it to the sub-folder.

Move by right-clicking:
Right-click the message, and click Move. Choose the folder from the list or click Other Folder if your folder does not appear.

Move by using the Move to Folder button:
Select the message and click the Move button. Choose the folder from the list or click Other Folder if your folder does not appear.

Referenced: Various pages on http://office.microsoft.com/en-us/outlook/default.aspx