I believe there was a setting in the text message application to change the size of the font. Try that first.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
This feature is able to be disabled if you open the Messages app and then tap the Menu button to access Message's settings. Just to be clear, Messages settings is different from your general settings, and it must be accessed from the Messages app itself.
tikibar1's answer is probably what is causing your message font to change size. You can disable the volume key controlling the font size. Open the Message settings and uncheck 'Use the volume key' in the Display settings.
Let's get your font back to its original size. Please go to Settings>Display>Font Size. You will be able to adjust it here.
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