JFB64 is right. The specs on the Sherline site will give you the sizes, but the power of the motor gets taxed the larger the diameter you are turning. They make spacer blocks to raise the head, tailstock and tool post which you can even theoretically double-up on, but at a certain point it gets ridiculous. The larger diameter gives the cutting tool more leverage to over power the motor.
I have turned 6" steel parts on my Sherline, but I needed to keep the cuts light to not stall the motor. I just had to take more passes than I would have on a large lathe. I was surprised how well it works on bigger parts, but a smaller machine just isn't going to have the grunt of a large machine. That being said, most of the parts I've ever made have been small - even on big machines.