Hi Florian,
It is simple to calculate the lot size as well as the amount of lots you can trade safely.
When you start with less than $500, you'll be able to risk anywhere from $5-$10 on any occasion (which is 1-2% of account balance).
With such modest limits, you won't be able to trade with Standard lots at all, and won't be able to trade with mini lots safely either.
So, having $500 at the start, you'll be looking to trade with micro lots (0.01 standard lot) regardless the leverage you choose.
1 micro lot equals $0.10 USD with xx/USD pairs, and currently around $0.11 USD for xxx/JPY pairs and same pretty close value - about $0.095 USD for xxx/CHF pairs.
What I want to say is that you can always count on 1 pip being equal to roughly $10 cents every time.
This immediately should give you an idea that while trading with 1 micro lot you'll earn (or can lose) $1 dollar every 10 pips.
If you open 2 micro lots, you will earn (lose) $2 dollars every 10 pips.
Ok, can you, for example, take 3 micro lots and risk $3 dollars every 10 pips, which will bring you $100 profit/loss in just 35 pips? (Thinking of risks rather than wins is a proper strategy; $100 is 1/5 of your trading account.) Considering the numbers, you'll see that 3 micro lots isn't a safe bet with $500 account.
That's where you start, and that's how you progress, adding 1 micro lot at the time as your account slowly grows.
Happy trading!
