According to its homepage, you call TinyBox, like so:
TINY.box.show('advanced.html',1,300,150,1,3);
This function takes 6 parameters: the HTML content for the box or the relative path to the AJAX source, a toggle (true/false or 1/0) if the content is via AJAX, the width of the window (use 0 for auto), the height of the window (use 0 for auto), a toggle (true/false or 1/0) to animate the window, and the time in seconds to wait before auto hiding the popup (optional).
So, it seems that if you set the third and the fourth parameters to zero, it should resize to fit the content.
EDIT:
In light of your comments, the following seems to be the case:
You are opening an iframe in TinyBox.
From within this iframe you want to submit a form.
The page this form is submitted to, should also be displayed in TinyBox.
TinyBox should resize to the height of the new content.
The three files involved in the above are index.php, new.php and new_1.php
The only way I could get this to work was by editing new.php like this:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>TinyBox demo</title>
</head>
<body>
<form id="myForm" action="new_1.php" method="POST">
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<input type="submit" name="submit" />
</form>
<script>
var form = document.getElementById("myForm");
form.onsubmit = function(){
parent.TINY.box.show({iframe:'new_1.php',boxid:'frameless',width:292,height:100,fixed:false,maskid:'blackmask',maskopacity:40})
}
</script>
</body>
</html>
Here's a demo of it working.