In veg only...I will look at the plant from above. If the leaves extend over the edges of the cup...I start thinking about transplanting. Making the lady rootbound on purpose isn't necessary, and can easily cause problems if let go too far.

In early to mid flower only...I'll look at the total volume of the plant, from the base...up. I envision wadding that plant up, and if I think she wouldn't fit inside the same size bucket she's in now without a trash-compactor...I'll transplant to the next size larger pot. Kind-of a stupid guideline I use for transplanting, but has worked well for years.
My guess is, there's a correllation between the plant's natural drip-line, and pot space. Between roots and canopy density and fullness.

Altho it's easy to dick-around with the plant, and to skirt these guidelines...it's served me well for years. Sure...you can grow a two foot plant in a 2-litre bottle...but are you providing the best enviornment for your plants? Are they providing the best results for you?