Unfortunately there is not a way to accomplish this crossbrowser with any reliability. There have been several previous threads in the forum that might help you with this issue: http://www3.formassembly.com/forum/search/?PostBackAction=Search&Keywords=limit+characters&Type=Topics&btnSubmit=Search and http://www3.formassembly.com/forum/search/?PostBackAction=Search&Keywords=limit+characters&Type=Comments&btnSubmit=Search
Happy to answer other questions, FormAssembly.com Support.