If the wheel speeds are steady, the robot's centre drives along a circle. How big is that circle? Forward speed v and turning rate ω together answer it. A point moving at speed v while its direction turns at ω radians per second moves on a circle of radius

R = v / ω

You can see it makes sense: fast forward motion (large v) with a gentle turn (small ω) draws a big circle; a slow crawl with a quick turn draws a tight one.