Every time the there is a character entered in textbox you will have to
check if it greater than the limit and if "Yes" then change the height
of the textbox. I have written a function for the same refer to it and
customize it your needs. I hope this helps.

Private Sub TextBox1_Change()
count1 = Len(TextBox1.Text)
If (count1 > 300) Then
MsgBox ("The Max length  you can enter is 299")
Exit Sub
End If
If (count1 > 200) Then
TextBox1.Height = TextBox1.Height + 20
End If
If (count1 > 100) Then
TextBox1.Height = TextBox1.Height + 20
End If
End Sub

Note : If there are more objects below the textbox then you will have to
change the position for below it so that textbox will not overlap on
other objects.