Capture its output using an output buffer:
imagejpeg($imJPGHandle, null, 100);
$obImageData = ob_get_contents();
ob_end_flush(); // use ob_end_clean if you don't want to display the image
Alternatively, you could write to a file, read it, copy it, and unlink it. Seems like too much work for my liking :P
To get the size, $obImageData (in my example) should be a binary string. I believe you can simply use strlen on it. If it doesn't work, I'll figure out a solution for you (can't test where I am).