Just remove the class atribute and and add size="10", it should look like this:
<input id="$customField.id" name="$customField.id" type="text" value="$textutils.htmlEncode($!value)" size="10"/>
Just change the number 10 with the number of caracters you want to show up.
I belive this documentation will help you achieve what you're looking for: https://confluence.atlassian.com/display/JIRA/Changing+the+Size+of+the+Text+Area+Custom+Field
Let me know if this information helps you!
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