Your hosting provider may have set restriction on uploads through PHP. The default install is (I believe) 8 megs, but it's not uncommon for those to be reset to significantly lower sizes.
Another possibility... The server you're on can't handle the load to do an on-the-fly install with the Zip upload. It's often an issue with free hosts, or shared servers that are "oversold".
Check with your host to get those answered.
As for documentation for manual installs -- If it's not at the authors site, or embedded in the zip file... Ask the author or post something here or at the authors site on the specific package you're installing. Some manual installs can be complicated and not for the faint of heart
