The Trade Size is the total value of your Trade at the moment it is opened. It is the same as the Amount that you put in, times the Multiplier.
For example, if you invest an Amount of £100 with a Multiplier of 5, your Trade Size is £500. This is also the amount for which you have to pay the Trade Fee.
If you have any further questions in regards to this, please feel free to contact us via email at BUX Support.