You'd either have to provide a value for the circle radius, r, or use the variable "r" for that purpose.
Draw a circle of radius r. In the first quadrant, draw a sample radius from circle center c to the circle. The angle between this radius and the x-axis is 45 degrees. Thus, you can find the x- and y-coordinates of one vertex using the following formulas:
x = r*cos 45 deg
y = r*sin 45 deg