What kind of bike is it?

Super skinny tires will have trouble mounting to wider rims, so you probably don't want 700x23.

Wider tires than 41/43 are often going to be MTB tires. There are fatter slicks but not many. The upper limit of what tire will work will be dictated by clearance in frame, brake calipers, fenders, etc.

What kind of surfaces to you plan to ride? If mostly dirt, get some knobbies. If mostly road, get some slicks.