DH session Key length

[ikuzar]

Hello,
I 'd like to know the length of DH session key generated by
DH_compute_key(unsigned char *key, BIGNUM *pub_key, DH *dh) . Here :
http://www.openssl.org/docs/crypto/DH_generate_key.html
It is said that *key* must point to *DH_size(dh)* bytes of memory. is 128
bits the default length ? how can I adjust this length according the
symetric-key algorithm I use ( AES128/ICM)
Thanks for your help.