Race skins come in 59mm or 62mm. The difference is glide versus grip. But (IMO) what you gain in glide with a 59, you more than lose with slippage. So I always use 62s.
More important than width is how they’re cut. If you end up racing a lot, you’ll end up with a fleet of skins for different conditions and terrain. In general, you want to cut them about 10-15cm behind the back of the heel piece. Start longer, test, and adjust. (It’s hard to start shorter and adjust… 🙂 )
If your skinning technique is good, I don’t think there’s much to be gained past 10cm. Some would argue you can cut them right behind the heel piece.
Here’s a photo of a very used skin and how it wore. The vertical line marks the back of the heel piece. As you can see, the plush is gone under the heel and very little is worn behind it. There’s some edge wear back to 10cm, and almost none at 15cm: https://photos.app.goo.gl/mnNxgCcnLgHgg3jy8
I hope that helps.