The size trait says "Size affects movement, how well the creature can hide and move undetected, and how easy or hard it is to strike in combat."

The table says "To Hit Modifier", it doesn't specify BS or WS.

I take it this means you get +/- to hit creatures at range and in melee based on size. So an ogryn will be easier to hit in melee, and a ratling would be harder to hit in melee. Am I reading this right?

Yes. The to-hit modifier applies to all attacks.

Awesome time to make a melee ratling.

I'm not sure about this. This question was addressed to FFG in the Deathwatch forums (different game I know), and the answer was that size affects only BS.

On the modifiers table on page 254 and in the "Size" entry on 255, it states that size modifies Shooting attacks. Nothing is said about melee.