Now in a pair where USD is the base currency ie USD/CHF,
1 unit = $1 therefore 100 000 units (1 lot) = $100 000.

The margin or deposit required to open a trade in this case
is dependant upon the leverage which your brokr will allow.

At Oanda the highest allowed leverage is 50:1,
therefore $100 000/50 = $2 000 must be deposited to open
a 1 lot trade where the pip value is $8.80/pip.

If you substitute a different leverage say 20:1, the deposit is higher,
$5 000, if 100:1, lower, $1 000. The pip value though always stays
the same.

So lot size/ leverage amount = Margin used (or deposit required) to open a trade.

All this is worked out for you in the market order window at Oanda,
just substitute different unit values then watch the numbers change.

When the EUR is the base currency the calculation is
100 000 x Eur/Usd exchange rate, expressed in USD.

When the GBP is the base currency the calculation is
100 000 x Gbp/Usd exchange rate, expressed in USD.

To make it simple for you to start with a new account and still you are not really fully understanding of the combination of lot and the leverage .
Just open the account with $300 euros and select 50:1 margin ( If have amout $1000 , should be more reliable and you can set lot of 10,000 to earn $1 for 1 pips movement) . Since with oanda you can open position with any amout . Just proceed with amout of 5000 in the lot size . For to increase amount , you need to proceed with first trade in order to accumulating pips . Ok , i give you one idea is to select your first pair select that pair with lower spread and that pair currently on the move in the a very long uptrend or downtrend. You select this pair and then analysis the 3 hour chart , set bolinger band , rsi , slow stochastic , ema ( when on the cross ) , macd to downtrend or uptrend. when all indicator showed overbought on uptrend or downtrend , enter your position accordingly with a stop loss at the overbought territory.