Does anyone know how much disk space and bar a standard ubuntu installation is on mongo? trying to map my VPS needs ...
There are no minimum requirements as such, but I would not recommend using Mongo in the same field as your web server.
MongoDB automatically uses all the free memory on the machine, as its cache is ( http://docs.mongodb.org/manual/faq/fundamentals/#does-mongodb-require-a-lot-of-ram )
This, along with the reasons mentioned in this answer (albeit for RDBMS), makes running mongodb on a separate box (even a small one) a much better idea.