The Print dialog in ArcGIS isn't set up to directly input pixel units. You have to do a little math and set two different parameters to get the output you want.
First, in the page setup, uncheck the box that says use printer paper settings and enter a value of 5 inches for width and height.
Next, when you go to export your svg, you need to do a little math and set the appropriate dpi setting. You might want to do the math beforehand to know how many inches to use. In your case you want a square 500 pixel image. So the easy math is to make the image 5 inches on a side and use a dpi (dots per inch) value of 100. 100 pixels per inch over 5 inches is 500 pixels.
It's odd that, as you can see in the first image, they offer a Screen page size, but even if you select that it doesn't enable using pixels for units. There's also a Slide page size, which defaults to 10 x 7.5 inches. The only way to really control output in pixel dimensions is to use the math between page size and resolution, setting both appropriately.
BTW, points is a completely different unit, mostly related to fonts. There are 72 points in an inch. See http://en.wikipedia.org/wiki/Point_(typography)