Are you sure you are trying to install the theme? The Site Shaper is a much larger file than just the theme and is intended to be installed through FTP; if you are trying to install that instead, then that might be the problem.
If it is the actual theme you are trying to install, then your PHP settings are configured to limit files uploaded through PHP to be fairly small. You can either have your host increase the limit, or you can use an FTP client like
FileZilla to upload the theme to your wp-content/themes folder after extracting it from the zip file.
Thanks Tristan. It was the theme, not the Site Shaper I was trying to install. I am aware of the upload limit but I have never had this problem with any other theme I have tried to upload, they just upload, activate and that's it. I tried some of your other themes as well and the same thing is happening.