$10/$20 (6 max) - Holdem - 6 players
Villain Is 34/21/2.39 over 2k hds

BTN: $748.05
Hero (SB): $342.00
BB: $190.00
UTG: $463.75
MP: $998.50
CO: $362.91

Hero posts SB $5.00, BB posts BB $10.00

Pre Flop: (pot: $15.00) Hero has 9 9

fold, fold, CO raises to $20.00, fold, Hero raises to $30.00, BB calls $20.00, CO raises to $40.00, Hero calls $10.00, BB calls $10.00

Flop: ($120.00, 3 players) 5 7 4
Hero checks, BB checks, CO bets $10.00, Hero raises to $20.00, fold, CO raises to $30.00, Hero calls $10.00

Turn: ($180.00, 2 players) 7
Hero checks, CO bets $20.00, Hero calls $20.00

River: ($220.00, 2 players) J
Hero checks, CO bets $20.00, Hero.. Is This Pot Too Big To Fold Here? What Range am l facing here?

I would call. You are ahead of >9% of his range.

I would put him on 66+, AJo+, ATs+, KTs+, and some other suited connectors/offsuit broadway cards.

At this level (10/20 online(?) 6-max) - his line of aggression doesn't signify only overpairs - he re-took initiative on the flop - this could be overpairs, but also overcards and straight draws (66, 88, or the other 99 combo). Since you have just called after a somewhat standard flop raise - he could think his entire range is good or has fold equity. If you do Villain's combos you will see his range is behind > 9% of your hand - and thus you need to call. This is a 'standard' WA/WB situation and merits a call based on pot size - expect to lose but have an overlay on your call.

Unless he's a psycho who is spewing with QJo and JTo on the small streets, your decision to call down is really made on the turn.