BuddyPress.org

The php function is substr()
There are many ways to use it.
Here’s some:
http://stackoverflow.com/questions/79960/how-to-truncate-a-string-in-php-to-the-word-closest-to-a-certain-number-of-chara

You mean the description field ?

That’s tougher. How good are you at php ?

afaik – you need to modify the
/themes/your-theme/members/single/profile/edit.php

You won’t see a form field for ‘description’
BP uses bp_get_the_profile_field_type() to show fields.
So you need to do an ‘if’ on the bp_the_profile_field_input_name()
And then use javascript to limit the number of chars allowed – lots of examples of this online.