//改成這樣
a = (int**)realloc(a, (size + 1)* sizeof(int *)); //(size + 1)* sizeof(int *)
還有,a[i]的長度不變,不需要再realloc,這樣就行了
for (int i = size; i < size + 1; i++) //只給新增的int*分配內存
a[i] = (int*)malloc(6 * sizeof(int)); //malloc