A very similar question was already asked here. Unfortunately, as it often happens, it was mistakenly marked as a duplicate of the following question.
While the latter is a perfectly legitimate solution, the former is asking how to modify a single block, which is a different question. Since the "duplicate" question doesn't admit further answers, I ask here again (mods: feel free to undo the duplicate error and merge both questions).
How to modify the width of a single beamer block, while keeping it centered?
See following sample image of what I want:
