Presumably you are thinking of fundamental strings...that's what's usually discussed in these forums...there are a different "brand" of string that makes up elementary nuclear particles...called QCD strings and those are huge maybe 1020 times longer. Strings can also be thought of as 1 branes...one dimensional (length only) branes...particles would be zero branes (no dimensions). So strings are a special class of branes. (I think this depends on the particular string theory.)
The former are supposedly near the Planck scale, so one dimension and largr than Planck scale, yet there IS a type of unity between the two types of strings. Leonard Susskind says "you could suspend 1040 trucks from a fundamental string"...(gotta love THAT measurement unit!) they don't expand (extend) very easily.
Susskind points out that black holes, even the supersized giants at the center of galaxies, DO have the energy needed to extend strings to immense size..such that a black hole might consist of a single string! As an example, one theory posits the horizon of a black hole as a single tangled string with quantum fluctuations...when a bump in the string appears on the horizon, it appears to an outside observer as an open string...ends connected to the horizon...and when one breaks off due to quantum zero point energy...carrying gravitational energy, voila, Hawking radiation!