You'll probably need to sand/file down the bottom/sides to fit; it unlikely just drop in unless you're buying an official Fender US Standard Strat nut for your Fender US Standard Stratocaster.
I'd personally measure the existing nut/neck first and then buy, to save ball-ache.
Then just measure the neck, and see whether the hole is curved or flat at the bottom. Otherwise I've found them to be fairly consistent, bar a little sanding. If you can I'd take the neck to the shop to get the closest fit.
