Introducing new kinks

This is a half success story and half advice post. So my wife is fairly vanilla in her sex life and I have my kinks which I have been introducing slowly. For Father’s Day we went on a date night on Friday since we have two kids and had a sitter. We got a nice dinner and I told her that I was taking her shopping afterwards. She was confused obviously since it’s Father’s Day. I drove us the 30 minutes to the “local” sex shop. And we just browsed for an hour or so. She ended up picking out a vibrator with clit stimulater which is fine and we got a few more small things and I slipped a starter set of anal plugs in. When we got home we had a lot of fun with the vibrator. Today we woke up we had a nice breakfast and took a nice nap after. When we woke up things got hot and heavy. I told her I wanted her to try something new and told her I would be careful and if she wanted to stop just tell me. It took a little warming up to the idea but once the smallest plug was in and then I asked if she felt comfortable she said yes. So I started to penetrate and the look of fire in her eyes was something I’ve never seen in the 7 years we’ve been together. I honestly thought it was hurting her so I stopped. That’s when she grabbed me by the hips and she thrusted me into her. It was amazing. I guess that’s enough bragging so I’ll get to the question. When do we know we need to step it up to the next size plug. I have ordered her a new starter plug to wear around the house as she works from home just need to know sort of the natural progression of sizing up