You may find many solution on the community for this, an example you can use script like below with onChange() client script.
var str = newValue; // var str = g_form.getValue('variable_name')
var len = str.length;
if(len> 10 && len< 5)
{
g_form.setValue('variable_name','');
alert('Please enter atleast 5 and maximum of 10');
}
Or you can Click on configure variable. Goto default Value tab-->in variable attribute field add below.
max_length=40 // OR set the limit value which you want to set.
