An order size refers to the entire purchase amount of an order. The minimum size of the order is periodically adjusted for each trading pair to maintain order sizes that are reasonably proportioned to their values.
You may obtain the minimum size and other details for all trading pairs through this API: https://api.bitfinex.com/v1/symbols_details.
For instance, to read the BTCUSD pair's endpoint, the minimum order size is the area marked in the figure below:
If you have any inquiries regarding the order size, please contact Bitfinex Support for further assistance. We will be happy to help!