I would stick with 18", I think 19 and 20 look funny...I have 18x10 in the front and 18x11.5 in the back with 275/35/18 front tires and 335/30/18 rear...I think the 18x10 in the front are a great choice. but I would of liked to put 18x13 in the back with 345/30/18 tires.. that is about as wide as you can go... The tire hight is just slightly higher than the stock tires, so it still looks good....
I have not lowered mine, but would like to one day...That will fill in the wheel wells....You just need longer bolts in the rear to lower the back, but the front is a little more complicated. You will need to remove the spring and and replace the wedges that sit on top of the spring...
Here is a picture of mine with the 18" wheels.
http://www.corvetteforum.net/c4/mikez40/