If the eductor has a 2½"female connection it can be hooked straight to a 2½"discharge (Hose or from truck). If the female side is a 1½" connection it must be hooked to an 1½" discharge/hose or to a 2½"-1½" reducer.
A foam eductor should be provided 200psi at the eductor's intake as per manufacturer's specifications. If the eductor is attached to a discharge on the pumper, the discharge pressure will be 200psi as shown below with a theoretical maximum of 300' of 1¾" hose after the eductor. 
General preconnects/crosslays on our apparatus are 1¾" hose so the general maximum will be 300' however, some specifications may only allow a max of 250' and combinations of eductors/nozzles may produce various limitations.
For simplicity and practicality, most maximum handline lays after the eductor will be 200' of 1¾" because that is what will be pulled from the crosslay.

The apparatus can become the exposure concern and may not need to be within 200' of the fire. In such cases 2½" is laid to the eductor, foam buckets are deployed away from the apparatus and foam handlines are extended from that point. The eductor still needs to receive 200psi so friction loss must be accounted for in the 2½". Engine 63's Akron Model 3072 will require a 2½"-1½" reducer to connect the hose.