Oops....my bad. Sorry I used Plus Sizings wrong earlier. Plus Sizings is just Tire Rack's name for adding an inch to your wheel diameter. For example, if your car comes stock with 16", then +1 woud be 17", +2 would be 18", you get the point. But basically they do all the math for you, knowing stock wheel+tire diameter, a +1 would yield the closest match to stock wheel+tire diameter, they tell you the tire size to use.
http://www.tirerack.com/wheels/tech/plusone.htm
I accidentally referred to it as meaning a width increase. For width increases, there are calculators out there or you can just do it yourself.
My car comes stock 195/65/15. I put 205/60/15 on it. I didn't want to go up to 215 since the wheel is only 6.5", 215 is better suited for a 7" wheel.
Personally, I like wide tires. If I get custom wheels I'll get 225's on the car...but I doubt I'll ever get wheels. The benefit to wider tires is performance, however there are drawbacks. With wider tires the car is more likely to hunt/dart on uneven roads, more likely to hydroplane, and more costly to replace when you turn 'em into baldies.
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Edit: added link