I in part agree with both answers above (by dick_freebird and ThisIsNotSam), and although not saying that explicitly, they clarified me (and confirmed my suspicions) about why there is no generic sizing tool out there, commercial or not.
Yet, the digital design spectrum is not composed by standard cells alone, right? There are other approaches that might be feasible, like regular fabrics, creating dedicated cells for specific projects and so on. I still think that a generic tool, if sufficiently parameterizable by user-defined cost functions, might save a lot of analysis and trial-and-error sizing.
In particular, we work with cell libraries for asynchronous QDI design, and there is no such library available supporting this kind of design style, except those that we did developed, such as
https://github.com/marlls1989/ascend-freepdk45, and others we cannot make open-source due to NDA signing. QDI design requires a set of cells including some large transistor networks (reaching several dozen transistors).
We managed to optimize the sizing of cells in commercial libraries using an in-house tool (according to some user-defined desirable performance characteristics), which shows me that for some specific need a fast executing, easily parametrizable sizing tool might be quite useful.