Forget "72 ppi". It has no relevance on the web - resolution is for print. Save For Web in any case strips resolution data.
Instead decide how many pixels you want, and always view at 100% in Photoshop (View > Actual Pixels or ctrl/cmd+1).
Thank you for your help. When I save my logo as a jpg it loses its transparent background. How do I resolve that?
You need to use a format that supports transparency and on the web these are only PNG and GIF. Still, since in particular palette-based transparency in GIF and PNG 8, has limitations, you might wanan consider baking your website's background color into the logo's surroundings and not using any transparency at all...
Or using PNG-24, which DOES support partial transparency in recent browsers.
Try resizing this page, which shows a centered PNG-24 image with a partially transparent drop shadow: http://Noel.ProDigitalSoftware.com/temp/TransparencyTest/TransparencyTest.html