What does DPI mean for a rendered page?
DPI sets how many pixels each inch of the page becomes. A4 is 8.27 inches wide, so 150 DPI gives 1,240 pixels, 300 DPI gives 2,480, and 600 DPI gives 4,960. US Letter is a shade wider.
Because area grows with the square, doubling DPI quadruples pixels and roughly quadruples the file. A 300 DPI page of text might be 400 KB as JPEG; at 600 DPI it is 1.5 MB, and on screen the two look identical.
JPG or PNG for a page of text?
JPEG smooths edges, which is exactly wrong for sharp black text on white: at lower qualities you get a gray halo around letters. PNG keeps every pixel and compresses flat areas well, so a text page is often smaller as PNG than as a high-quality JPEG — and cleaner.
For a page that is mostly photographs, JPEG wins comfortably. For a mix, 300 DPI JPEG at high quality is a fair compromise.
When should you not render at all?
If the goal is to send the document, keep it as a PDF: images lose selectable text, searchability and links, and a page image is usually larger than the page it came from.
If the goal is the photographs inside the document, extract them instead — you get the originals rather than a re-photograph of the page.
Why did a big page come out smaller than I asked?
Browsers cap how large a single canvas can be. An A0 drawing at 600 DPI would be around 20,000 pixels wide and exceed that limit, so the render is scaled to the largest size your device can actually produce.
Split the drawing into sections first if you genuinely need that resolution.