If you are open ended, then it has to be a fairly wet board. It may not be soaking wet like 3 to a straight or a flush, but it is at least a little wet.
The bet sizing really depends on a lot of factors. 1. Position 2. Stack sizes 3. Level of the tournament 4. What is your image 5. How do your opponents act and react. 6. Are you drawing to the nuts.
If you have position and it is checked to you, I am generally in favor of starting to build a pot unless you think you are going to get check-raised off of your hand. This is why it is important to know your opponents. If you think they are check-callers or check-folders, then you can make that bet and either pick up the pot right then or play to make your hand (and potentially
bluff if you miss and you only put your opponent on something marginal that you can get to fold).
If you are OOP, then it is generally wiser to check-call or even consider check-raising depending on what hand range you put your opponent on.
These of course are just general guidelines, I have not broke down all of the factors I listed. You should ponder on them all and decide how you want to proceed on a hand by hand basis.