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?
In my experience so far, an app should be a reusable entity. There many guiding principles for choosing what to go and what not in an app. Separating authentication is one example. A projcet is one big collection of may apps and a reusable app can be in many projects. 
Nowadays, there is a trend to move to micro service architecture, which is next level of separation of functionality with each service doing best one thing.
Each project is a unit in itself, not an individual app. So you host the project, not the app. I recommend using different project for each site and using micro-service architecture. A lot depends on your existing codebase too.
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.