I have upgrade/migrated my oracle Database from (windows 2003 standard edition) 10.1.0.3.0 standard one to (windows 2003 enterprise edition) 11.2.0.3.0 standard one ,in 10.1.0.3 I was using pool size 30 but after migration I was unable to connect, I changed pool size to 50 it worked for some time and faced the same issue, now I have changed to 100 now its working , I didn't understand the problem and why I have to keep on changing the pool size,for your reference i have providing the setting details, please help me to solve this issue.

<property name="hibernate.connection.driver_class">oracle.jdbc.OracleDriver</property>
<property name="hibernate.connection.url">jdbc:oracle:thin:@***IP***:***Port NO***:***DB NAME***</property>
<property name="hibernate.connection.username">*****</property>
<property name="hibernate.connection.password">*****</property>
<property name="hibernate.connection.pool_size">100</property>
<property name="dialect">org.hibernate.dialect.OracleDialect</property>
<property name="show_sql">true</property>