Hi ceblanda,
300em just an example to shows you the setting work, you do not need to set it to 300 em.
Please try the code below
<%@ Page Language="C#" ValidateRequest="false" %>
<%@ Register TagPrefix="CE" Namespace="CuteEditor" Assembly="CuteEditor" %>
<html>
<head>
<title>example</title>
</head>
<body>
<form id="Form1" runat="server">
<CE:Editor ID="Editor1" runat="server" Height="15em">
</CE:Editor>
<div style="height: 15em; border: solid 1px red; width:400px">
test</div>
<br />
</form>
</body>
</html>
It will look like
Regards,
Ken