MysqL_select_db("yourdatabase");  
$q = MysqL_query("SHOW TABLE STATUS");  
$size = 0;  
while($row = MysqL_fetch_array($q)) {  
    $size += $row["Data_length"] + $row["Index_length"];  
}

然后转换为兆字节：

$decimals = 2;  
$mbytes = number_format($size/(1024*1024),$decimals);