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
It would help narrow things a great deal if you'd provide details like whether or not you intend to distribute the database along with your software; your system will be hosted; how much data; etc. 

Don't assume anything with regard to licensing. Get a lawyer, maybe even one who specializes in open source law.

"...regardless of disk space..." - capacity always depends on this. Where do you think the data goes? Better to think about things like sharding your data, RAID, clustering, replication, etc.

I would worry about any system whose developer had to come to a forum like this to ask that kind of question. You should have people on staff with sufficient skill and knowledge to have a strong opinion on this sort of thing.