The paste operation is a drawback of validation.

You could use an event macro if you can get someone to write the code (I'm not much of a programmer).

Here's how to use conditional formatting to highlight cells with more than 20 characters.

Let's assume you want to apply this to the range A2:A10.

Select the *entire* range A2:A10 starting from cell A2.
Cell A2 will be the active cell. The active cell is the
one cell in the selected range that is not shaded. The
formula will be relative to the active cell.

Goto the Home tab>Styles>Conditional Formatting>
Manage rules>New rule>Use a formula to determine
which cells to format

Enter this formula in the box below:

=LEN(A2)>20

Click the Format button
Select the desired style(s)
OK out