To size a flare, we will need to know, available pressure (at flare inlet), Max Temperature of the stream, Average molecular weight of the stream (Yet better, flare gas composition), and flow rate (Mass flow rate is preferred) for each stream. (In short, Pressure, MW, Temperature and Flow rate as a minimum).
Other information, could be the plant’s limits (radiation limits and other environmental requirements), preferred structure type (Self-supported, Guy-wired or derrick supported)