Click on the View tab
Then click View Settings.
On the Advanced View Settings dialog click Other Settings, then you can change your column or row font settings.
To change the font or font size for the column headers in your Inbox (like “From” and “Subject”), click Column Font, and then make the desired adjustment and click ok.
To change the font or font size of the message subject lines in your Inbox, click Row Font, and then make the desired adjustment and click ok.
Change Font and/or size as desired and click ok
Click OK and OK on the Other Settings and Advanced View Settings screens.
The above instructions modified the folder view you were on (ie: the Inbox). If you want the same look for all your folders you need to copy these changes to all your other folders. You can do this by following these steps:
Click the View tab, then click Change View, and from the dropdown choose Apply Current View to Other Mail Folders
On the Apply View dialog box, choose the folders you would like to apply the view to and click ok. If you want to apply the view to all the folders, make sure the top most folder is chosen (should be your email address), then click the box to Apply view to subfolders and click ok.