Question: * What's the limitation?
* Is there a specific volume of data each can handle regardless of disk space?
* When to use what assuming licensing is not a problem?

Answer: 
This is a very nuanced question that really cannot be easily answered, as each situation can provide many pluses and minuses. Also, MySQL being owned by Oracle now and several branches off of the main functionality means that MySQL != MySQL anymore.

If you are looking for really really big data sets, then you will like have to break with the RDBMS sets and start to look at things like MapReduce and other large data set processing technologies.

I have personally worked with all three over the past decade or so from the application perspective. They all have their advantages, like MSSQL working will with the other Microsoft technologies like LINQ where as MySQL having a large open community support and Oracle being the workhorse of the commercial sector with lots of ability to embed application logic right into the database.

Again, it really depends on the application, the situation, the skills of the people who will maintain it after it is developed, commercial considerations, hardware and platform considerations, etc etc etc.
It depends what you are trying to do and obviously it has to do with cost.

* MySQL and Postgres are very widely used by a huge number of startups because its open source and there is a lot of support out there for people using it
* MSSQL is good if you are using MS programming languages because of the ease to connect and use.
* I have never used oracle but know people use it a lot for data warehouseing so can't have that much of a bad name

All of these will suffer from similar issues when scaling because they are RDBMS databases. They do also have decent ways to get round it and with a decent ORM used in your code then it shouldn't matter what you use.

Pick the one that all the developers are comfortable with