In the first case yes, in fact it would be exactly that, a pop up window:
HTML Code:
<a href="http://www.yourdomain.com/index.html" onclick="window.open(this.href,'','width=300, height=250');return false;" target="_blank">Link Text or Image Tag</a>
In the second case, no. Theoretically it can be done but, there are so many and different security features in play on the various browsers that in reality it is not.
You can have a splash or front page from which links like the one above are available.
If a user doesn't have javascript enabled, they will get a plain new window or plain new tab, depending upon their browser and its settings. It can also be done so that if a user doesn't have javascript enabled they will get nothing.
None of this is really that great of an idea though. Pop ups should be reserved for special effects, not a site's main page. Designing a page with the idea in mind that it will be rendered in various sized viewports is the way to go rather than trying to control the user's viewport. Once you take control of the user's viewport, you are alienating many people as well as making your page perhaps inaccessible to some users whose browsers cannot conform to your settings.
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