There is no definite answer to this question but based on recent practices all around the industry in the big data field, the entry-level criteria for data to be considered as big data is around 1 TB.
Frameworks like Hadoop are used for this purpose. It can be noted that big data analytics can be used for smaller databases as well when mathematical operations are concerned. R and Python are also in wide use when integrated with Big Data technologies for the same purposes.
To gain more knowledge of big data, sign up for this big data online training.