This error is caused by your hosting account for your domain not being set with sufficient upload allowance. OptimizePress is around 3mb in size, and some hosts set a default of 2mb. If you have access to your web hosting control panel you may be able to increase the minimum upload filesize yourself, or if not please contact your host and advise them to increase the size to 5-10mb.
Alternatively, you can use FTP to upload the theme to your wp-content/themes folder inside your WordPress installation.
This error is in no-way caused by OptimizePress so we will always refer you to your web host to fix this issue as it's out of our control.