Downsizing a shirt is difficult - depending on whether you want to reduce the length, the width or the sleeves etc. You would need to go to an alterations place and get a quote. There's a lot of moving parts in a shirt. Please note that downsizing the collar will be near impossible for a ready made shirt for anything reasonable in price.
The easiest and cheapest would be to reduce the sleeve length only. For the rest, I would just live with it as a roomy shirt.
Quite a few dry cleaners offer an alterations service for simple changes like sleeve length. Might be worth checking out your local.