We really don't care how big your enter link is, as long as it is clear. A tiny text enter link on an empty page is fine, as is a full screen gif.
The only time we care is when we can't find it. This is often the case with enter links that aren't in the obvious place (ie. at the bottom of the page). If it's a small text link, it can get lost amongst the clutter. If it's an image saying "Click here for porn pictures", we might think that it's an ad rather than the enter button.
I know I've seeen sites where I've spent over two minutes hovering over various links and images trying to find a way into a site, only to give up and delete it. How many surfers do you think will spend any more than 5 seconds looking for the entrance?
