The P365X uses the standard P365 slide, and the P365XL frame/grip module.
You want to use the standard size tactical application rail, as the XL rail will be too long for the slide. (link to product page below)
Tactical Application Rail for Sig Sauer P365
If you intend to use a compensator with your P365X please consult the 'Choose Your Rail Length' chart on the Pro Ledge Tactical Application Rail page.