how can I get size of data in BLOB?

I store pictures in mysql BLOBs.
1) when I use something link this:
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$res=mysql_query("select myPicture from pictures where id=12",$link);
$row=mysql_fetch_array;
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what type will be variable $row['myPicture']?

2) how cat I get size of $row['myPicture']?
3) does "readfile($filename)" function keep $filename file opened until
client get it? i.e. if I have myDoc.pdf (2MB size), execute
"readfile("myDoc.pdf")", and client recieve this file with 2kb/sec speed.
Does it mean "mysDoc.pdf" will be open about 1000 seconds?

With best regards,
                  Igor Mikhailov.