Will the picture be stretched or squashed?
No. The proportions are always kept, whichever of the three ways you give the size: a width of 1000 on a 4:3 photograph produces 1000 × 750 and nothing else. Changing the shape of a picture is a different operation and it is called cropping — there is a page for it.
Why will it not make my picture bigger?
Because enlarging cannot add detail. A photograph 800 pixels wide holds 800 pixels of information; asked for 2000, any tool can only invent the pixels in between, and the result is a heavier file that looks softer than the original. So the request comes back saying what size your picture really is. If you need the larger canvas anyway — for a print layout that demands one — the option is there and it says what it is doing.
Which size should I ask for?
For anything going on a screen, the width is the number that matters: 1920 is a full desktop background, 1280 fits any web page, 800 is generous for an email. For an upload form with a size limit in megabytes rather than pixels, use the image compressor instead — it searches for the quality that fits under a size you name.
Does resizing remove the location my phone recorded?
Yes, along with the camera model, the lens, the exposure, the timestamp and any editing history. The blocks that hold them are cut out of the finished file, and the file is then measured: the result says how many bytes of that data were in the picture you gave it and how many are in the one you get back. It is done that way rather than left to the browser because browsers disagree — Safari copies those blocks into a re-drawn picture and Chrome does not, so a tool that assumed the re-draw was enough would be telling the truth on a laptop and not on an iPhone.
Is the picture uploaded to be resized?
No. The decoding and the redrawing both happen in this browser tab, and there is no server here to send anything to. Open your network panel while you use it, or disconnect from the internet after the page has loaded — the tool keeps working.