There is no official metric for defining what “big government” is. It is a term typically used by Republicans to refer to any aspect of government that they think is unnecessary or overbearing.
There are some measures that can be used to quantify the size of the federal government. One of these is the size of the civilian workforce employed by the federal government. Another measure that captures more of what government does (other than employing people) is the level of federal spending as a percentage of the country’s GDP. A final measure is the number of rules passed by the government in a year and/or the economic impact of those rules. Between them, these metrics give us a good deal of information about the size of the government.