A good rule of thumb with repeat customers is that they should be placing larger order over-time (Average Order Value increases). That would be a sign of growing trust and loyalty in your customer base.
Some products might be durable with a consumable component (e.g. printer and ink). In those cases the durable product might need to be ignored. Same for products with a steady consumption amount (e.g. 30 day supply).
If you don't see an increase in order sizes, first check if repeat customers are ordering more frequently. It could be you're getting smaller orders from repeat customers that make-up for the drop in order sizes. You want to look at the time between orders (Customer Purchase Latency).
If none of those are true and your order sizes are dropping, that's a symptom of a loyalty problem. Repeat Customer Insights will automatically notice and flag those issues for you. There's also a bunch of ways to improve your AOV for repeat customers.