I would resize them manually. While you can resize them with opengl I'd be concerned that the results wouldn't look as good. Depending on what you are doing it may not matter.
Viewing post in Question about manual sprite assets management
But that is not what the creator of the topic is asking about. He is talking about a image one pixel taller being noticed. If you are noticing that squishing it down by one pixel or the others up by one pixel may not look good. He can obviously just try it but I would rather do than manipulation by hand.