I completely agree with these levels, but disagree with mapping them to “small, medium, and large.” That’s a lossy compression if you will, and the heart of the problem. A shorthand might start with good intentions, but rapidly is muddied by conflicting interests.
+ That's trivial, it will be ready for testing before lunch.
+ I know how to do that, should be ready tomorrow/next day.
+ I can see how to do that but there are lots of other constraints - at least a week, might be more.
+ It's a big project, needs more planning and specification before it becomes a series of estimable tasks. Let's do that.
+ That breaks other things we care about. We need to prioritize and be ready for rework.
If you want, you can call these small, medium and large.