There are many different ways. Perhaps one of the simplest:
sql_query = SELECT product_id, product_name, \
GROUP_CONCAT(CONCAT(param_name,'_',param_value)) AS params \
FROM products LEFT JOIN params USING (product_id) \
GROUP BY product_id \
ORDER BY NULL
(Thats actully mysql query, but sql-server cant be that much different)
Can then just do (SphinxQL) queries like
SELECT * FROM index WHERE MATCH('keyword @params (Size_Small | Color_Red)');
(if any of your params have spaces, might need to use REPLACE to change to understores, to keep them as one word - or use "phrase syntax")