Houjichasan

size factor < 0 -> large (from what I remember, measured in terms of stock price * num of stocks outstanding).

value factor < 0 -> "growth" firm (defined as low B/M value).

hard to generalize whether a mature company would have size factor < 0 and value factor > 0. A lot depends on its relative size (for size factor) and particularly industry (for value factor).

You can directly read a paper by Fama and French on "Multifactor explanations of asset pricing anomalies" for example.