I think you have a few misconceptions about these vector graphics formats: PDF, EPS and SVG all provide a way of specifying the explicit size of the graphic. In SVG, there are two overlapping mechanisms: defining an explicit width and height, and the viewbox, which defines a coordinate system and the aspect ratio for the SVG data.
It’s quite possible that the way Dorico handles SVG isn’t perfect in this regard, and it’s certainly true that it would be helpful to be able to specify an explicit scale factor for a graphic in a frame, rather than relying on the current fit behaviours, but adding EPS support wouldn’t provide a magic solution.
EPS has been effectively replaced by PDF at this point in almost every conceivable workflow, and in any case it cannot be supported in Dorico for the same reasons PDF cannot.