It depends on what line was drawn by inkscape. You can open the SVG file with a text editor (notepad or something more readable) to inspect it.
One possibility is that inkscape added a path (https://developer.mozilla.org/en-US/docs/Web/SVG/Element/path), in that case, the line is just encoded as a string, and you can modify that string as a whole, but it’s quite hard to parse a generic path to get out the end points.
The other possibility is that inkscape added a line object (https://developer.mozilla.org/en-US/docs/Web/SVG/Element/line), in which case the endpoints are simply encoded as
(x1,y1), (x2, y2), which should be very easy to modify.
In any case, it’s possible to write SVG files by hand. F.e. the SVG below is a line from the top left corner
(0,0) to the bottom right corner
(100%, 100%), and it will scale to whatever size you want in Ignition (can even be used for horizontal and vertical lines.
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
viewBox="0 0 100% 100%"