In general, the diameter of the circle should be approximately three times the height of the skater. To set your scribe to the right length, calculate your height in inches and multiply by 1.5. A skater who is five feet tall should use a setting of 60 inches times 1.5 = 90 inches, or a little more or less depending on personal preference. Judges seem to like big circles, so if you’re unsure, make them bigger. This table of figures circle sizes will spare you the math.
For loops, the diameter of the circle should be approximately the height of the skater. That means if you are five feet (60 inches) tall, you should set your scribe to 30 inches. Generally, standing with your feet as far apart as they can go without too much difficulty is about the right radius; you can pivot around one foot and draw with the heel of the other to get a reasonably close loop circle. The loop itself should be about one blade-length wide and 1.5 blade-lengths long.
Roller skaters use standard figure circles painted on the floor. For circle figures (the big ones), the standard diameter is six meters (19 feet). For loops, the standard diameter is 2.4 meters (eight feet).