My approach would be to think in terms of risk.
Ask yourself 'how much am I willing to lose'; and remember you must always accept the fact and be comfortable with losing.
Let's say you ok risking 1% of your account with a potential trade setup, then you have the amount in cash and therefore you can calculate the lot size. There are many lot size calcs on the net and I have one setup in meta4. Just remember to think in terms of risk/capital management, I have seen too many people blow their accounts because of poor risk management and/or not understanding lot size.
Trade safe.