Search:

For what you want I think 4/8, 9/16 would work. Just a little trickier to use than 5/8 so slightly hesitant to advise it as a best answer. I still think 2 razors is ideal.
For honing. ...

Will you shave your whole neck or just trim the beard there ?

If you shave the whole neck you can use the biggest razor you can handle. Trimming is another story.
Strictly for trimming you could...