Hi Elizabeth,
What I usually do is approximate the area or box I want to draw and then I complete the sketch. After I have this, then I SEND TO RHINO by right-clicking on the selected sketch. I get the same curve near the origin (or depending where the sketch is). I use BOUNDINGBOX to get a nice rectangle. In GH, I right click the sketch again and say LOAD FROM RHINO. Once you do this, select the new rectangle. This version of GH will not update the sketch unless you drag the sketch around.