A standard (or medium) width last is designed to fit the majority of riders with average foot width. If you've never had issues with boots feeling painfully tight across the ball of your foot or so loose that your heel lifts, this is likely your best starting point.
The benefit of a standard last is that it offers a balanced fit—snug enough for good board response and heel hold, but with enough room to stay comfortable all day. It's also the most widely available width, giving you the biggest selection of models and brands to choose from.
If you're unsure about your width, get properly measured at a shop—foot width is independent of length, so you can have a long, narrow foot or a short, wide one. Always try boots on with your actual riding socks, and if a standard width feels cramped or causes numbness, you may need a wide last instead.