How to export selection as svg using extendscript?

If you are using the `app.activeDocument.exportFile()` method, it will export at the current document size. You could try fitting the current artboard to the selected artwork and then exporting (see below). FYI, this will get weird with multiple artboards, so you would need to account for that edge case (and probably a few more).

(function () {
    // get the active document
    try {
        var doc = app.activeDocument;
    } catch (e) {
        alert("No active document.\n" + e);
        return;
    }

    var fp = Folder.desktop + "/export.svg";
    var ab = doc.artboards.getActiveArtboardIndex();
    doc.fitArtboardToSelectedArt(ab);

    var exportOptions = new ExportOptionsSVG();
    exportOptions.embedRasterImages = true;

    var type = ExportType.SVG;
    var fileSpec = new File(fp);

    doc.exportFile(fileSpec, type, exportOptions);
})();