In your textViewDidChange method, call
[tableView beginUpdates];
[tableView endUpdates];
This will trigger heightForRowAtIndexPath to recalculate all your cell sizes each time the user types a letter. Then in heightForRowAtIndexPath, you can then calculate the necessary size for your cell.
The sizeWithFont method will return the size needed to display the text in a UILabel, which is slightly different than that for a UITextView due to content insets, line spacing, etc. I've used a somewhat hacky solution in the past. If you create a temporary UITextView, set it's text, and use [UIView sizeThatFits:constraintSize] to get the size that will fit all its content within the constraints. (The documentation on this method is a little unclear - take a look at this answer for more info: question about UIView's sizeThatFits)
UITextView *temp = [UITextView alloc] initWithFrame:someArbitraryFrame];
temp.font = DISPLAY_FONT
temp.text = cell.textView.text;
//This gets the necessary size to fit the view's content within the specified constraints
CGSize correctSize = [self.temp sizeThatFits:CGSizeMake(CONSTRAINT_WIDTH, CONSTRAINT_HEIGHT)];
return correctSize.height
In interest of efficiency, you should probably store/lazy-load the temporary textView so that you're not creating a new UITextView for every cell, but rather re-using the same one to calculate height for different text.