What is a Lot?

A Lot is the standard transaction size, which measures the value of the deal.

The range and the size of lots are dependent on the asset selected and can always be found in information:

The lot size effects the required margin needed to open a position. 

A) Lets say we initially were to open a 1 lot trade on Apple (stock) which is equivalent to 100 shares.

B) With a leverage of 1:20 (highlighted in yellow), the trade would cost US$1,018.25 (highlighted in red).

C) If the Required Margin is too low or too high, the Required Margin can simply be raised or lowered in the LOT size section, with fraction of lots also being available. 
As seen below, the lot size was lowered from 1 lot (100 shares) to 0.1 lot (10 shares).

D) The required margin is now $101.83 (highlighted in red).

All minimum and maximum lot sizes available to trade can also be seen in "information"as seen below:

CFDs are complex instruments and come with a high risk of losing money rapidly due to leverage. You should consider whether you understand how CFDs work and whether you can afford to take the high risk of losing your money.