Help! Post flop raise sizing.

My post flop raise sizing sucks. Pre-flop I am now happy with as decisions are of course more straight forward but post flop I might know why I am raising but not know really best amount to raise to?
There are many types of raises and I would think that the amount/type of raise employed by more experienced players would vary dependent on opponent type, opponents hand range, stack sizes and whether hero has made hand/bluff/draw.
Any advice would be appreciated. Here are some examples from some recent sessions.

1. Villain only raises PF with premium holdings (20/4)

Full Tilt Poker, $0.50/$1 NL Hold'em Cash Game, 5 Players
LeggoPoker.com - Hand History Converter

SB: $357.30
BB: $91.90
UTG: $186.15
CO: $33.40
Hero (BTN): $105.65

Pre-Flop: 9 8 dealt to Hero (BTN)
UTG raises to $3, CO folds, Hero calls $3, 2 folds

Flop: ($7.50) 6 7 A (2 Players)
UTG bets $5, Hero raises to ?

2. Villain is fish (55/3). Donk lead is weak made hand or massive strength.

Full Tilt Poker, $0.50/$1 NL Hold'em Cash Game, 4 Players
LeggoPoker.com - Hand History Converter

UTG: $109.10
Hero (BTN): $100
SB: $76.25
BB: $36.25

Pre-Flop: J 7 dealt to Hero (BTN)
UTG folds, Hero raises to $3.50, SB calls $3, BB folds

Flop: ($8) 9 A T (2 Players)
SB bets $1, Hero raises to $4, SB calls $3

Turn: ($16) 4 (2 Players)
SB bets $6, Hero raises to ?

3. Villain and I have tangled a few times. He is check raising my flop cb with air/flush draw alot of the time here.

Poker Stars, $0.50/$1 NL Hold'em Cash Game, 6 Players
LeggoPoker.com - Hand History Converter

Hero (BTN): $110.50
SB: $136.80
BB: $117.50
UTG: $65.30
MP: $99.50
CO: $95.15

Pre-Flop: 9 9 dealt to Hero (BTN)
2 folds, CO calls $1, Hero raises to $5, 2 folds, CO calls $4

Flop: ($11.50) 4 8 3 (2 Players)
CO checks, Hero bets $8, CO raises to $16, Hero raises to ?

1) 19
2) Make it 11 or something on flop. As played, 21-23
3) Anyone think this could be a CiB guys?

1) $18, reraise or call any shove until the money is in
2) Raise flop bigger, $7. As played I like $24
3) this is a shitty spot because of the pf action. if you've seen him do this before, i move it all in. if this is the first time, I probably just call and evaluate turn. If it weren't for the PF limp/call I would raise to like $40