Currently, grain size is absolute. Grain size relative to image dimensions would be a great idea. Theoretically, the grain size would also adjust relative to cropped dimensions much like the post-crop vignetting works.
Actually, it seems grain size is absolute. Sometimes we want to apply an equal amount of grain to photos, but a setting size of '25', for example, will look smaller on a 22MP file than it will on a 10MP file.
When i say it's subjective, im referring to the initial decision about appearance...
And yes, grain size is absolute, so a setting of 25 will look the same, regardless of pixel dimensions. If i'm not mistaken, the size slider is a direct translation of pixel size, so if you have an image which is 1000x500px, a setting of 25(px) will have the exact same grain size as an image which is 5000x2500px... they'll both be 25px.