There are a few ways you could handle your situation.
The first approach would be to alter the Sub GenerateSelectMULT_NOVALUES subroutine in shop$db.asp and change the line of code:
%><select multiple size="5" class="txtfielddropdown" name="<%=selectname%>"><%
%><select multiple size="25" class="txtfielddropdown" name="<%=selectname%>"><%
Save and re-upload your shop$db.asp file to your server to see the changes.
Since you are loading 10 thousand products into that cross selling field it will slow down the load speed of the shopa_addproduct.asp page. The second option you might like to take would be to set the configuration setting "xproductaddsimplemode" to "YES". This will replace the cross selling select list with a simple text input field where you just write the catalogid's of the products you want to associate with the current product. For instance, you would add something like 1, 3, 5 into the cross selling field to associate catalogid's 1, 3 and 5 with that product. This approach should speed up the process of loading the shopa_addproduct.asp page but will require you to look up the catalogid's from an alternative source.
Hope this helps.