SVG describes scalable geometry, while PNG needs a finite pixel grid. The export must therefore choose both width and height. VectorPNG reads a positive viewBox first, falls back to unitless or px dimensions, and refuses to guess percentages or physical units.
Aspect lock derives one side
With a 2:1 viewBox and width 1,200, the locked height is 600. For ratios that do not divide evenly, the derived side is rounded once to the nearest positive whole pixel. The result labels that rounding instead of claiming an impossible fractional pixel.
Unlocked dimensions mean intentional stretch
An unlocked 1,200 × 630 output uses that exact rectangle even when the source ratio differs. This can be useful for a deliberate graphic treatment, but it changes geometry. It is not a crop, contain or smart layout operation, so inspect circles, text and line weights before reuse.
Verify the receiving workflow
Open the PNG in the software that will use it and compare the important edges, text, colours and transparency with the source. Browser engines can shape fonts or colours differently and PNG stores fixed pixels rather than scalable geometry. Keep the SVG as the editable source; use the raster export only for the declared destination size.