Tire size I can't rightly say.
I would go with the OEM size for the 18s that come with the G6 stock.
hopefully someone can shed more light.
If you're looking to fill the wheel gap as best as you can then I would lower it first, measure everything and decide from there.
As for lowering the main option is lowering springs. Megan racing makes Coil-overs for the Saab 9-3 that will also fit our cars, but that's a grand for the parts only...
But as for lowering springs Eibach makes a set, Canuck Motor Sports makes a set, and there's some "Blue" springs on ebay that I'm not too sure about. All of which will lower you about 1.5-1.75 inches, The canucks will lower a bit more than the eibachs.
Here's how mine looks lowered, I'm on SSC springs (they're essentially canucks)
Also in these pictures I have wheel spacers attached with the stock 17's, the spacers bring it closer to the edge and in turn the tires look closer to the body.
My front is a little higher than others also, this is because my 4 banger is a bit lighter than the V6's out there.