Downsize is a common Instant card from Return to Ravnica (RTR), collector number 38. Current market price: $0.09 (foil $0.14), TCGplayer market prices via Scryfall.
Target creature you don't control gets -4/-0 until end of turn. Overload {2}{U} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")