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 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.