I'd also suggest a cross bike.
You'll have more options, both with bike models and in versatility of use, than with a dedicated "gravel racer."
On finely crushed gravel, I've done OK with 28c's. Wider tires will let you use lower pressures, which can improve comfort but (for various reasons) may be a little slower than 28's, which in turn will be a little slower than 25's or 23's.