PC Gamer has a nice tutorial explaining how to take a screenshot of your entire Stardew Valley farm.
Since the farm in the game is so big and there is no in-game function to screenshot the entire farm, this requires installing a utility, GeDoSaTo, that is designed to modify how game graphics render.
Using GeDoSaTo, PC Gamer explains how to run Stardew Valley at a whopping 8K resolution and then downsample that so the image is viewable on whatever resolution you’re actually running the game at. From there, GeDoSaTo will let you take a full 8K screenshot of the entire farm.