Hi Grumpy,
In warmaster (fantasy) all chariot must be in short edge frontal (20*40mm), so I guess you should trimm horses in your model.
In Warmaster ancient, heavy chariot and scythed chariots are a basing exception to the normal basing convention , because they are so big they are based on 40*40mm (double base).
Warmaster ancient appendix 2 dealing with base sizeing gives you the follow rule just in case you switch from 20mm to 40mm front edge. It says add+1 attack modifier when charging and pursuing. Note that is the same as the standard rule for Shock Cavalry mounted to the long edge.
Hope it helps