SVG is refused by most social networks, marketplaces and document editors. AVIF gives the smallest files of any widely supported image format. SVG is at home in logos, icons and illustrations that must scale; AVIF is the usual choice for the smallest images for websites.
The defaults suit most files. Under Options you can set the quality, aim for a target file size, resize the result and set the scale the vector is rendered at.
The SVG is drawn at its own width and height (taken from its viewBox if it has no size) times the scale set under Options, scaled down if that would pass 16,384 pixels a side or 64 megapixels. Scripts inside the SVG never run and files it links to are not fetched, so text in a font the SVG does not embed is drawn in a fallback font. The AVIF encoder is a WebAssembly module that downloads the first time you convert to AVIF and is cached afterwards.