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The ~4/8K would be a good (IMHO) choice since you can do a great deal with it and it keeps the cost down, if that's an issue. Add CrOx to whatever backing you wish and you have a pretty basic but...

No, they'll be every bit as good but you will have to do more 'strokes/laps' with the razor. They aren't *that* small so they'll be perfectly useable but if you plan on honing razors often, you might...

Depends how often you plan on using them - more than occasional use and you may regret not saving more and buying the larger sizes. I'd be inclined to pick up a 4k/8k combo stone or there abouts if...