Winforms Circular Checkbox
you should use auto properties where you don't need any validation. So all of your properties should use them like instead of


private bool isChecked;
public bool IsChecked
{
    get { return isChecked; }
    set { isChecked = value; }
}



you would use  

public bool IsChecked { get; set; }

this example of really unnecessary tenary expression 


IsChecked = IsChecked ? false : true;



should be replaced by  

IsChecked = !IsChecked;  

using braces {} for single if statement also would make your code less errorprone  
you should extract your magic numbers into some meaningful aka good named constants ( 3.0f , 1.5f etc)  
you should extract "\u2713" to some meaningful constant  
you should extract new SolidBrush(tickColor) to a field, as the brush itself won't change.