To determine whether your Mirabelle is fit for carting, you should obtain orthopedic clearances on her. Orthopedic soundness is the critical consideration, rather than size.
Size will certainly dictate how much she should pull (up to her own weight), but if she does not clear her hips, elbows, and shoulders, she should not be involved in this endeavor (and will not be allowed to participate in AKC or BMDCA competitions).
At two years of age, you can get OFA clearances done (
www.offa.org) or you can have PennHip clearances (
http://www.pennhip.org) done now on her hips which is a more accurate, scientific based clearance (the OFA has a panel of three radiologists review and grade and so it is more subjective). If you do PennHip on her hips, you should still get OFAs done on her elbows for sure since Berners are so highly prone to hip and elbow displaysia.
Good luck!