Is-there-an-equivalent-to-PosSizer-class

Yes, it's called PositionSizer. Check the "Support" link above, Extensions API and you should find the documentation for it.

It looks like BasicPositionSizer is the one.

The BasicPositionSizer is a convenience when you need to extend a standard position sizing type (fixed dollar/share or percent of equity). Or you can use the PositionSizerBase if you don't need these options because your position sizing method is self-contained, for example, like Fixed Ratio or Percent Volatility.