Restrict a field to only 100 characters

I want to restrict a field to only 100 characters. For example, the Summary of Change field... There should not be more than 100 characters when user provides a brief summary. Is it do-able?

The easiest way would be to use an input validation formula like the following...
@If( @Length(fieldName) > 100 ; @Failure("Field too long, blah blah blah") ; @Success );

Only problem with that approach is that the user will be alerted when they try to save the document instead of when they are editing the field.

Using LotusScript and the field's exiting event, you could also check with something like the following...

Dim workspace as New NotesUIWorkspace
Dim uidoc as NotesUIDocument
Set uidoc = workspace.CurrentDocument
If len( clng(iudoc.FieldGetText( "fieldname" ) ) ) > 100 then
     Msgbox "Failed", 0, "Failure"
     Uidoc.GotoField( "fieldname" )
End if

Note: If the field the user goes into has an exiting event that also puts the cursor in another field, you could have trouble (i.e. the exiting events could be fighting over where to put the cursor)

You could also use Javascript to check as you type. If you're not familiar with JavaScript, go to javascripts.com and you'll find samples.