A vector has no fixed size, so the width you type is the width you get, with the height following the drawing’s proportions. The box starts at 1024 pixels. Set it to 0 and a size read out of the file is used instead — its width and height, or its viewBox. When the exact size matters, type it rather than leaving the box at 0.
This page loads no libraries at all. The work is done by the browser itself. Your file is read on your own machine and never sent anywhere. Open your browser’s network tab and watch while you use it.
Whatever width you type, with the height worked out from the drawing’s proportions. Set the width to 0 and the size is read out of the file instead, from its width and height, or from its viewBox. Those two do not always agree, and which one is used depends on how the file was written, so type a width when the number matters.
Then the viewBox is used. With neither of them the file states no size at all, and you get one of two things: your browser’s own 300×150 default, or a message saying the file has no size. Neither of those is your drawing, so add a viewBox to the file before converting it.
A percentage is not a size in pixels, so it is skipped and the viewBox is read instead. The same goes for a width given in em, pt or mm. Only a plain number, with or without px after it, is read as a size.
Yes. Nothing is painted behind the drawing, so whatever is transparent in the SVG is transparent in the PNG. A JPEG could not do that. It has no way to store transparency, and those areas come out white.
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