ORIGINAL: ParkerTrailblazer
The size is for carbon arrows.
Carbon arrows come in different sizes depending on weight/grains per inch, so there really isn't a "one size fits all for carbon arrows."
Also, if you've had it for a while and this is a recent problem, it may be that the whiskers are becoming worn out and need replacing. Especially, for example if you're shooting a stiffer fletching like Blazer vanes which will cause more wear on the whiskers. This was an issue I had with a WB in the past. I was shooting Blazers and practing (a bunch) and it wore the whiskers out pretty fast.
Like others have said, though, make sure alignment & center-shot are correct as well.