Python Vs Ruby On Rails : on Size
One option with Ruby on Rails is to go with a JRuby deployment which would allow you to pack it all into a single .war file. This would require the person deploying the web application to have a java web application server (Jetty is probably the smallest and easiest to bundle).
With Rails, you are generally going to have to install Ruby and any required ruby gems. The Ruby install is going to be machine specific- different for Windows/Linux. Everything else should be easily scripted. If you go with an Apache Passenger (mod_ruby) solution, you will need to get that installed as well. 
In reality, I haven't run into many server applications with simple, compact installs.
I am planning to do a small web application that will be distributed as a single installable. I have plans to develop this application in either Python/Django or Ruby On Rails. (I am a Java/C++ programmer, hence both these languages are new to me). 
My main concern is about the size and simplicity of final installable (say setup.exe). I want it to be small in size and also should be able to pack all required components in it. 
Which one among Python/Django and Ruby On Rails is suitable for me?