要在JavaScript中设置警报框的宽度和高度，您需要使用自定义警报框。此警报框的样式为CSS。
使用以下代码设置警报框的宽度和高度，该代码使用JavaScript库jQuery-
<!DOCTYPE html>
<html>
<head>
<script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js">
</script>
<script>
function functionAlert(msg, myYes)
{
var confirmBox = $("#confirm");
confirmBox.find(".message").text(msg);
confirmBox.find(".yes").unbind().click(function()
{
confirmBox.hide();
});
confirmBox.find(".yes").click(myYes);
confirmBox.show();
}
</script>
<style>
#confirm
{
display: none;
background-color: #F3F5F6;
color: #000000;
border: 1px solid #aaa;
position: fixed;
width: 300px;
height: 100px;
left: 50%;
margin-left: -100px;
padding: 10px 20px 10px;
box-sizing: border-box;
text-align: center;
}
#confirm button {
background-color: #FFFFFF;
display: inline-block;
border-radius: 12px;
border: 4px solid #aaa;
padding: 5px;
text-align: center;
width: 60px;
cursor: pointer;
}
#confirm .message
{
text-align: left;
}
</style>
</head>
<body>
<div id="confirm">
<div class = "message">This is a warning message.</div><br>
<button class="yes">OK</button>
</div>
<input type="button" value="Click Me" onclick="functionAlert();" />
</body>
</html>