FlightXML is intended for recent, in most cases up to two weeks, flights. During this time range flights can be found in FlightInfoEx by submitting the carrier and flight number as the “ident”. While IATA can work, ICAO carrier is strongly recommended. The faFlightID found in the FlightInfoEx responses can be used in AirlineFlightInfo to learn gate and terminal information.
Pricing information is detailed here. Duplicate web requests are considered separate charge events.
For historical data extending back over a year you may want to consider a data report instead.
Size and cost is dependent on the amount of activity per identifier, these values can be previewed along with included data fields in the order forms. Format is TSV or CSV. More specific questions are best handled via the contact forms on those pages.