The minimum trade size is dependent on the asset class.
In many cases, one unit of the asset is the minimum size, such as one barrel of Crude Oil or one share of Apple.
For major currency pairs, such as GBP/USD, the minimum trade size is 500 units of the base currency. Please note, when using leverage, the margin requirement will be lower than this.
An error message (as shown below) will appear on the web trading platform, if you attempt to trade below the minimum size.