If I give a wrong key size (56 for example) keytool says that 2048 is a
valid keysize. But if I give 2048 as the keysize it does not accept.

Here is a way to reproduce.

TestingKey Sizes .....
                        COMMAND = keytool -genkey -alias first -keyalg DSA -key
size 56 -sigalg DSS -dname cn=Satya -keypass keypasswd -keystore mykeystore -sto
repass storepasswd
java.lang.Exception: Invalid keysize. Keysize must be equal to 512, 768, 1024, o
r 2048
                        Key size = 56 : test PASSED
 
COMMAND = keytool -genkey -alias fourth -keyalg DSA -ke
ysize 2048 -sigalg DSS -dname cn=Satya -keypass keypasswd -keystore mykeystore -
storepass storepasswd
java.lang.IllegalArgumentException: Modulus size must range from 512 to 1024 and
 be a multiple of 64
                        Key size = 2048 : test FAILED command unexpectedly fail
ed