What's the best sizing rule of thumb?

Does anyone have a reliable guideline for getting the right size?

I discovered today, quite to my dismay, that the CB-3000 I recently bought is too small. It goes on, but it pulls significantly. My anatomy leans more toward width than length, and now I’m concerned about purchasing another device that doesn’t fit properly. Any advice would be appreciated. Thanks!

Could you clarify what "pulls a lot" means? I've used a CB-3k for over ten years. I just want to ensure I grasp exactly how your situation feels.