Well, technically, any size possible can be used. But that's not typically best practice, and will make your life harder. It's not about "even" numbers, it's about multiples/powers of 2. Maybe a practical example would make this clear: 1) Two wall modules, one with length 256 and one with length 512. Those two pieces will…
Another option is to make the door frame line up on the power of 2 grid while having the hole for the door a different size. You can see an example of that in Thiago's door frame model.
As said before, take values in between. You don't need to stick with power of 2 values only, but with multiples of your grid/a low power of 2. Regarding your example, a 64 Grid would allow for a doorway with 192x128 dimension.