Have you talked to your girlfriend about this? Is your girlfriend upset that you don't like the size, or is she upset that you won't wear the shirt (and you haven't told her why)? Because if it's the former, that probably warrants a larger discussion.
Not even getting into why this question isn't on the AITA subreddit but I swear most of these AITA questions could be solved by basic communication instead of asking some internet strangers what they think
naturalheel t1_j2b0a2u wrote
If the shirt does not fit, you must acquit.