I am getting big coals in my wood stove. What does this mean? Improper burning?

Break the coals open, if they are burnt all the way through then its fine, if they are unburnt in the middle then your burner is inefficient, try letting more ari into the burner by opening the vent. if you burn wood and coal together then the wood will absorb all the heat and the coal will not burn so well. if these solutions fail you could just try breaking the coals up into smaller pieces so that they burn all the way through.