The maximum cutter diameter for the OF 1400 is listed at 63mm. At that size, the maximum safe speed would be 17615 rpm which equates to approximately speed 4.
For reference, the OF 1010 is 50mm and the OF 2200 is 89mm.
Obviously you need to take into account different bits of various types when considering size along with how much material you are looking to remove in each pass. Plenty of people use the OF 1400 for flattening slabs though, so it's certainly doable.