I recently tried to update to os6, and when the download of new software was supposed to start, I got an error saying that I should check my internet connection. The connection is good, tried to update through blackberry website, same problem again - couldn't download the new version of software. When I tried to download the update and then install it, halfway through the download I got a prompt saying that the size of the file is unrecognisable...any ideas?

Try deleting Desktop Manager and reinstalling the latest version from RIM's site:

BlackBerry - BlackBerry Desktop Software

Then download a fresh copy of the latest OS version for your device from the sticky at the top of the 9300 forum. And make sure you're downloading the version for your exact device. Don't use a 9330 version on a 9300, etc.