Novices and newbies discussion

It’s happening at last. I’ve been working with a domme for several months. What started as basic training from someone keen to get me off on the right foot has turned into an unexpected relationship for both of us. She absolutely loves the concept of keeping me locked up—starting with play sessions—but we’re both wondering about long-term confinement and denial down the line. I plan to buy a good quality cage. She owns one she picked up impulsively years ago but never really used; its diameter was too small, plus it had razor-sharp edges on some bars that made it practically unusable anyway. Could someone explain the correct way to measure so I don’t waste time and cash on a cage that either won’t fit or isn’t ideal enough to keep? Cheers!