Keep Coding

i have used
md5(sha1(rand()))
this will give me 2 levels of encryption and output hash with 32 length like
cbc7a20f14eab9262efe0adb21074d0a
but how i can let hash be 255 length (for example) ?

Whatever you're doing just stop it now.
Calling the same or different cryptographic functions repeatedly does not do anything like give you multiple levels of encryption. If anything, it weakens the encryption by making it easier to find a hash collision.
Moreover, neither md5 nor sha-1 are cryptographically secure.
If you're trying to create passwords, use bcrypt or PBKDF2 (and only once). Don't forget to use a unique salt and high iteration count.
You may use a KBKDF (key based key derivation function) like the ones specified in NIST SP 800-108 or HKDF to expand the output of bcrypt or PBKDF2 into more bytes. Note however that this does not increase the security level to the number of bytes in the output.

If you want to create cryptographic random numbers, anything relying on rand() won't do any good. No matter which hash or KDF you throw at it.
Use mcrypt_create_iv ($size, MCRYPT_DEV_URANDOM) instead, which conventiently has a $size parameter, solving your length problem at the same time.