no spaces in the selector :
$('tr:nth-child(2)[Chk_RPQSelection_col="1"]').length
will find all TR's that are the second children of a table looking like
<table>
<tr Chk_RPQSelection_col="1"></tr>
<tr Chk_RPQSelection_col="1"></tr> <!-- this one -->
<tr Chk_RPQSelection_col="1"></tr>
<tr Chk_RPQSelection_col="1"></tr>
<tr Chk_RPQSelection_col="1"></tr>
</table>
<table>
<tr Chk_RPQSelection_col="3"></tr>
<tr Chk_RPQSelection_col="1"></tr> <!-- and this one -->
<tr Chk_RPQSelection_col="4"></tr>
</table>
EDIT:
to count TD's with that attribute inside the second TR, you can do:
$('tr:nth-child(2)').find('td[Chk_RPQSelection_col="1"]').length