What's the cone spanner size for cone nut on Shimano WH-R550 Front wheel ? Is it 17mm or 18mm ? I tried with 16, it seems, need more, 1 or 2mm bigger.
EDIT: I took a late look at some pictures of the wheel you mentioned (Shimano WH-R550). It looks quite different from the regular mtb shimano hubs, so please consider the following answer as applying to regular mountain-bike shimano hubs.
From my experience with shimano hubs, there are two parts:
Some shimano models have a double-size cone, to allow use of two wrenches of different sizes, so it helps folks who have only one size of cone wrench (Shimano is rarely so considerate).
I waited (and suffered) too much before buying my cone wrenches. If I knew how much a difference they make, I'd have bought them at the same time I bought the bike!