I can't testify to the size but I just looked through the instruction manual.
On page 24, although it doesn't state the size of the wheel collar, it does say to drill the wheel with a 3/16" bit.
http://manuals.hobbico.com/gbg/gbga0063-manual.pdf
You can also ask the manufacturer
http://www.carlgoldbergproducts.com/...ort/index.html