I don't have a lot of experience with wheels or tires, but I plan on upgrading my wheels when my stock tires wear thin. I would say to try to stick to tires with the same outside diameter as factory as this will not mess with clearance height issues or rotations per miles variance.
Everyone has differences in opinions. But it is your CT and you should get something that looks good to you on your CT.
The width of the tire should try to reflect how aggressively you drive and much snow you get (if any) in your area and try to find a width that meets somewhere in the middle. Also note, there is a direct linear relationship between the width of your tires and the amount of road noise they will pick up. Well, to be fair.....there is also the same relationship with tire width and traction.
There is a good website to calculate tire size upgrade options here.
Also, remember to note the load index of the tires you buy. The CT is heavier than your average car, though lighter than many crossover vehicle on the market today, so the tires need to be able to carry the weight of the vehicle and its cargo. Selecting the width of tires is important in another aspect in that manufacturer sometimes limit tire selections based on the width alone. Ex: Lots of tire choices in the 245, but less in the 255 width. Good thing is a given set of wheels, will "allow" for varying widths of tires, only up to a certain point
For me, I like the look of the HFP rims on my CT. Like the one below. Before purchasing any wheel, try to Photoshop the wheel into a side picture of your car and see how it looks i.e. "it's stance" Nice wheels can look terrible, if it doesn't fit the overall image of the car, in its class and direction. Wheels are the #1 thing, that states the attitude of your car. Next might be front grille.
Hope this helps.