They can get quite large. THe first year I began tracking bimacs (2003-4), I recorded three large bimacs. One was my own, Ollie. I didn't expect her to grow so large, but she weighed more than three pounds at the time she died (I had kept her about 10 months).
It was difficult to measure the length of her arms because she could stretch them out so far, but she could sit in the middle of the tank on the bottom and touch the far corners of the top.
You can get an idea from the pic how big she was - that's my hand.