Key Length & Hashing
Short answer: just truncate, it's fine.

Long answer: you want a Key Derivation Function. A KDF turns an arbitrary-sized input (the shared secret obtained from SRP) into a configurable sequence of bytes, which you can split into as many sub-sequences as you need for symmetric cryptography. For instance, SSL/TLS defines a KDF (it calls it "PRF"; see section 5) so that it can extend the "master secret" (the shared secret obtained from whatever key exchange mechanism has been used) into six elements (an encryption key, a MAC key, and an IV, for each tunnel direction).

If you have a "reasonable" hash function such as SHA-256, and your secret data needs an amount of at most the hash output length, then a simple truncation will be an appropriate KDF. (Theoretically, being a good KDF after truncation is not implied by the hash function being appropriately resistant to collisions and preimages, but in practice you will not get any trouble with SHA-256 or SHA-512.)