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: 
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)
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.