This will depend on your timeframe.
But if a stock triggers a buy my own philosophy is that I want to buy it.
So Ill place orders at market--same when I sell--at market.
But to your idea.
The longer the trading timeframe the wider you can go as your return to risk is generally larger.(If you dont know your return/risk (and most dont) then you should keep records to determine it.)
But I would be using a 14 day ATR and initially say a 1.5 deviation as an intitial buy maximum.
It really is an arbitary thing.
You could also use say a % of your stop.
EG if you have a 10% stop on purchase price here is an example on a $3.50 stock.
Stop 35c from buy price
Maximum buy 17c from buy then raise stop to 35c from buy.
Can you see what I mean?