transmitter output power is considered as a "given" in microwave design. it is usually found on the microwave transmission equipment's specs sheet.
things you will have to compute..
FSL (depends on the hop distance)
line losses (depends on how long the transmission line will be, number of discrete bends if any, transitions)
antenna gain required (depends on output power, rsl required, fade margin and the sum of all losses)
min rsl required can be found on the specs sheet
from the computed antenna gain, you can derive the antenna size.