if you are having this problem you have the whole web design thing ALL WRONG.
the web is not a giant ( or ever whole) image uploaded as an HTML documents but a collection text and images.
You could probably use an image editor such as Photoshop to reduce the dimensions of your PSD document. (IMAGE> IMAGE SIZE)
but really what you should be thinking is what is the most efficient way way to cut this up into slices. Look for repeating patterns, and unique elements within the design then cut them out as separate image components and code your site to repeat and align those images as background to proper HTML elements ( which will ofcourse, contain your text ( atual TYPED text, not from the PSD)
hope that helps