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
Battery, Power Cell, 3000mAh (NiMH, 7-C hump, 8.4V) | Traxxas
GOLDBAT 11.1V 5000mAh 3S 50C Lipo RC Battery Pack Hard Case with Deans T Connector for Traxxas E-maxx Axial RC Car Truck Buggy Truggy Racing (2 Packs), Battery Packs - Amazon Canada
Traxxas EZ-Peak Plus 4S Completer Pack 6700mAh LiPo's TRA2998
Battery, Series 3 Power Cell, 3300mAh (NiMH, 7-C flat, 8.4V) - Canada Hobbies
2996X - Traxxas EZ-Peak Live 100W Multi-Chemistry Battery Charger (TRA2971) with 1 x 5000mAh 14.8V 4Cell 25C LiPo Battery (TRA2889X) - AMR RC
Inside the FASTEST Ready-to-Race RC Cars in the World! | Traxxas