You'll notice a link similar to this one:
Google resizes your photo to 288 pixels width. But that's pretty small, I hear you saying. The nice thing is that you can change the width, but only to fixed values. Here's a list of numbers you can use to replace 288:
72, 144, 200, 320, 400, 512, 576, 640, 720, 800, 912, 1024, 1152, 1280, 1440, 1600.
If you want to place a direct link to a photo, you can use 1600, as this seems to be the biggest value.