Need help resizing photos - confused as to why this isn't working
You can use save for web, just don't forget to turn on clip to Artboard. Otherwise items off the artboard would make your document larger. An artboard not filled short of the EXACT edges, would also make your document smaller.
If you need a different format, use export but check use arboards there and then choose screen resolution.
AI thinks in inch and the default DPI setting for any new document is 300 DPI.
I cannot find that be true.