The maximum allowed file upload in most any PHP install by default is 2mb. Unless you alter your PHP settings on the server you are not going to get past that.
If the image is too big the server will drop it
As far as a white page timeout that can happen too on large images using GD2 its very common. You can try this suggestion but you are still bound by my discussion about the image size and php's restriction on max filesize. http://www.photopost.com/forum/1258964-post2.html