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
Traxxas 3.7v 650mAh LiPo Battery Alias | Mark Twain Hobby Center