[Question] Expand or Extend Size Modes

I have a usecase for a new size mode `MIN_ABSOLUTE` where the calculation is:

```
target[i] = parentSize[i] * this.proportionalSize[i] + this.differentialSize[i];
var absolute = this.absoluteSize[i];
if (absolute > target[i]) {
target[i] = absolute;
}
```

I can put in a PR and easily add this to the existing types...
_OR_
Should `Size` be opened up to allow custom size modes to be included? I am more in favor of this approach and will work up a PR for the custom size modes. But thought I would see if there are any opinions on the matter.