I do not use the validators myself, I do not like them because asp.net otuputs js with them and it outputs a bit of invalid code. Here is what I would do, I would put in the script that you are executeing on click of that command button at the bottom, as buntine said, and if statement
if len(yourtextbox) < 3 and len(youtextbox) > 5 then
<code to be executed if the textbox length is how you wanted it>
lblerrormsg would be an asp lable right after your text box.
Text="this feild is required<br>"