Is a django project only meant to be for a single website or can the different apps be different sites. What is perferable and advisable?
An app is a submodule of a project which contains functionality (views, models, urls etc) for a specific part of the larger site and is as decoupled from the other apps as possible. The project as a whole is the website and your apps make up the separate parts of functionality for your site.
If your sites are going to basically contain the same functionality, it might be worth looking into the Sites framework which django provides.
So taking the polls example further.
Lets say the website is a survey site. There would be the polls app, which would contain the relevant models and views for creating and recording poll results. Then you might need to view the data, so you could create an analysis app, which would store the views for displaying dashboards and contain functions for data processing. Then we could take things a bit further, and have users be able to log in and see their own results (and give us the chance to link users to poll results), so you would make an accounts app, which would hold views for logging in/out, maybe a profile page etc.
So each of these different parts of functionality would be separated out into distinct apps, which would make up the project (site) as a whole.
If the apps have been decoupled properly, you could reuse the different apps in other projects (e.g. the accounts app could be dropped into a new project do provide logging in/out functionality)
I have been using django for a while, but there is something im not quite clear on. 
How big should a django app be. If for example a django app should only be user authentication or if it should be an entire website in one app.
If I have a project with several apps and each app is a whole website with a lot of code, is that the way it suppose to be or should all apps related to a single site within a project ?
Im thinking of creating one django project for each site, but im now wondering if I should be creating one project where each app is one site. Can anyone please comment on this, what it the preferred way to do it ?
In the django documentation one app is only used for a poll, so it seems to be that, according to the documentation, that each app should be some part of functionality on the site.