Custom the button CSS styles.
<style type="text/css">
.mc .l-btn {
width: 100px;
}
.mc .l-btn-text{
line-height: 40px;
}
</style>
And then apply it to the confirm dialog.
$.messager.confirm({
cls: 'mc',
title: 'Confirm',
msg: 'Are you confirm this?',
fn: function(r){
if (r){
alert('confirmed: '+r);
}
}
})