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
Traxxas 71054-8 Red 1/16 RTR XL2.5 E-Revo 4WD Monster Truck with Batte — White Rose Hobbies
Large ESC Mounting Plate for Traxxas 1/16 Scale Mini E-Revo Brushless | eBay