I don't think there is a way to set this, I've had a play about with the suggestion on this site: http://www.mydigitallife.info/never-combine-icons-or-only-combine-when-full-but-hide-labels-in-windows-7-taskbar/, however that only appears to affect labels on active programs.
I will continue to have a little bit of a Google and if I find anything, come back to you.
UPDATE
I've been having another look at this and found some more information for you, one thing is an update on the link above, the other is a workaround.
So the first part: http://www.thefreewindows.com/3693/change-the-size-width-of-taskbar-buttons-on-windows-7/
I will warn you that this requires some changes to the registry, so do so with caution and make sure you have a backup :)
Anyway, I suggest, try putting in the key that they suggest; MinValue (String Value) under HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics
Then make the value of the key something between 38 & 50. This might help. I would say the max you would want to give it is 500 really, that would take up a lot of space on your taskbar.
If that doesn't work, have a look at this link: https://social.technet.microsoft.com/Forums/windows/en-US/53fcc34d-19e9-48aa-849e-5db820ab6154/is-there-a-way-to-always-show-labels-on-all-buttons-on-the-task-bar?forum=w7itproui
This won't give you labels on inactive programs, however there are some options you can use.
One that I always use is to put the Desktop toolbar onto the Taskbar (right click the taskbar > hover over Toolbars > Click Desktop).
This will give you a menu option on the right-hand side of the task bar, like this: https://i.stack.imgur.com/6AYVl.jpg
I know it's not an exact answer, however I think that this is the best option you have short of having to use third party software or making changes to the underlying system files / settings.