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.
I'd say if you want to compare apples to apples, then it is MySQL vs SQL Express, vs Oracle Express.

Or if you have $, then it is the MySQL support license, MS-SQL Standard, vs whatever Oracle's cheapest offering is.

In my experience, once you choose a language, e.g. Php goes best with MySQL, then you've chosen your DB. Java goes well with Oracle. C# goes well with MSSQL.

Similarly, if you choose your OS, then unix flavors run MySQL or Oracle, but MSSQL is windows only. MySQL and Oracle work on both unix and windows of course.

[If you need to buy many machines, then not having to pay OS licenses for the server helps in scaling](https://blog.codinghorror.com/scaling-up-vs-scaling-out-hidden-costs/).