Sometimes we need Helptext with our field but in salesforce there is a limitation that you can define only 255 characters for the helptext (while creating a field).However in case if we need more than 255 characters we  can go with 2 options.
1)Create custom Helptext tooltip using jquery and javascript.
2)Another one is using helptext attribute of apex:pageBlockSectionItem

Here is the Information about 2nd option.
Use <apex:pageBlockSectionItem>, it has a helptext attribute that corresponds to the balloon. You can put your “long help” in a custom label for example so it can be translated to other languages like standard help texts.

<apex:pageBlockSectionItem helpText="Lorem ipsum ... (400+ chars)">

    <apex:outputLabel value="{!$ObjectType.Opportunity.fields.CurrencyIsoCode.Label}" />
    <apex:outputField value="{!Opportunity.CurrencyIsoCode}" />