99 on BB hits Four-of-a-kind on the flop

The sizing does not matter much. If he is folding, he is folding on any size...

In general, you can bet lower if you want weaker hands to call. However, this particular situation won't save you from that. Just bet 1/2 psb and if fold, move on.

Great spot to lead the flop small.

Would not be exploitative to lower the bet sizing? Or I am overthinking this thing here?

You are overthinking it.

Either check-raise flop, and keep on betting or donk flop and keep on betting. The river bet should be all in. Adjust sizes so that the first raise/bet you make is percentagewise (relative to the pot) the biggest, and the river bet percentagewise the smallest. Alternatively, wait until turn, but then apply the same principle.

The flop check-raise is probably most effective against the average player. Regs are more susceptible to the flop donk (they get mad because donking is fishy and may raise you to punish you for donking, "making draws pay" or "read you for a standard bluff on a paired board" or whatever they might think). Delayment 'til turn is risky, but part of a balanced strategy. Don't take the line you took. It is decidedly not optimal. Since you wait until turn with it, you need to get a good chunk of your stack in if you want (you should!) to get all in by the river. This is rarely possible with a turn donk.