Biggest batteries I've found to fit in the stock XRT battery tray are these HRB 9500mah 4s 120c packs. The Lectron pro 5200's the hobby sold me are way too small. These
Best Budget Battery for Traxxas X-Maxx 8S - Ampow Blog
Traxxas X Maxx 8s With Batteries Best Sale, SAVE 45% - abaroadrive.com