What size should I choose?
The size it will be shown at, or twice that for a screen with a high pixel density. A logo for a web page header is fine at 512 or 1024 wide; a poster for print wants the width at the printed size times the printer's dots per inch, which is why the cap sits at 8192. The height is worked out from the file's own proportions, so nothing is squashed.
Is the transparent background kept?
In PNG, yes. JPG has no transparency at all, so the transparent parts have to be painted a colour — white or black, your choice — and the result says which. If the picture is going onto a coloured page, choose PNG.
Why does the result say something was removed?
Because the file contained script, an event handler, or a reference to an image, font or stylesheet on another site. Those are not part of the drawing, and fetching one would be a request from this page to somewhere else, which this site does not make. What was removed is named so you can decide whether the picture is complete; a file whose whole image is an external photograph will come out without it.
Can I convert PNG to SVG instead?
Not here, and not honestly anywhere without a warning. Turning pixels into vectors means tracing shapes and guessing, and the result is only good for simple flat artwork. This tool goes the other way, from instructions to pixels, which is exact.
Is my file uploaded?
No. The SVG is read, checked and drawn in this browser tab, and the site has no endpoint that could receive it. Diagrams and logos are frequently the unreleased kind, and a format change does not need a copy of them.