Flopped nut straight, then saw a scary turn

[Murs316]

Say the pot is at about $300... If you are making a block bet on the turn, what is a good sized bet?

[NickPoker]

Is a middle pocket pair out of the question? I think that is just as likely as a set. When you checked you showed weakness, so he may be trying to steal the pot (or think he is good with a lesser hand than you). I think you made a mistake by giving up control of the hand, I would have bet the turn for around $100, and see what he does. If he pushes you have a tough decision, but I still may call.
What is your table image? Maybe he put you on a draw.
In my opinion he has a range of hands of:
PP 22 through 10/10, excluding 4/4, Flush draw, or A5s.