This board is pretty good for AA or KK

It's hard for your opponents to hit this board almost ever. I would cbet half pot with your blocker to one of your opponents have a KK.

However if your opponents don't think much of your 3betting range or your cbetting range given the game flow, I would consider a xb because they may be more likely to make a move on you which is hard to fight back against unless your very certain that they are for sure making a move

I usually bet like $10-$15 on those boards if BB and CO are (somewhat) reasonable. BB will x/f tons of rundowns and shouldn't coldcall a lot of KK. CO has to x/f a lot of unpaired hands as well and I also expect a decent amount of x/folds from pocketpairs since he's  in a shitty spot vs higher pairs OTT & OTR.
Also, BB shouldn't have too many 5's. He's pretty loose so he prolly has more 5678 stuff than a snug reg however he's still folding lotsa 5s and due to card removal it's unlikely. CO is actually more likely to have a 5 since he opened in CO (hands like QJT5ss, low rundowns...) but again card removal and stilll a lot of trashy 5*** folding pre makes this def a good cbet. 

Saying all of this I honestly think that there isn't a very big difference between betting $10 and $20. While there might be some x/calls from hands like QQ if you bet very small in the vast majority of pots there will either be x/f by unimproved unpaired hands or x/c / x/r by 5s. Saving yourself the 10bb outweighs (imo) the times that you get a pocketpair to fold that would call a smaller bet.

I'd go for 13-14$ cbet for reasons above mentioned 

"what about board 255 with flush draw?"

On a board with a FD vs. 2 players who have shown some strength (i.e. are likely to have suited or double suited hands) I would definitely weak semi-bluff bet less often & probably reserve semi-bluff betting for hands with a FD in them (like AKT7 with a FD)

cbet sizing to $10 or $15

Good point Suniram that sizing a cbet to 30% saves us some bb's when we run into a 5. I thought for a sec that this might be best line but then I also thought that it may be more likely to get played back at & invite ourselves to getting bluff raised which is hard to play back against even at the top of our range with AA (I mean virtually the top of our range, like 1% of the time we do have trips or a boat). 

If you have experience with your opponents & know they won't pull a bluff raise on you, then I'd be very happy to bet 30% pot & fold to a raise but without experience or with a read that your opponents may be tricky/aggressive, I feel a 30% bet invites playbacks & turns our hand face up when we continue (if we call, we have AA or KK & we fold all others bluffs). What do you guys think about this?