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: 
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
As to skaffman's point you may want to have a look at postgres if mysql isn't scaling for you. It is a more mature and robust than mysql and is opensource. The time to make the switch is highly dependent on you application environment, however, if you need clustering and replication to work properly 100% of the time then postgres will not let you down (as mysql has for me in the past)