(Trivial) The problem is very well understood, no extra investigation is required, the exact solution is already known and just needs to be implemented, no surprises are expected, and no coordination with other teams or people is required.

(Small) The problem is well understood and a solution is outlined, but a little bit of extra investigation will probably still be required to realize the solution.

(Medium) Features that are well understood and relatively straightforward. Bugs that are relatively poorly understood and may not yet have a suggested solution.

(Little more than Medium) Features that are well understood and slightly complex. Bugs that are loosely understood and without a suggested solution.

(Large) Features that are well understood, but known to be hard. Bugs that are very poorly understood, and will not have a suggested solution.