The amount of available resources on board a spacecraft determine the extent of its potential participation in Federated Satellite Systems (FSS). FSS are a new approach to space missions with the goal of enhancing mission sustainability, performance and reliability by the exchange of resources such as data storage, processing and relay among participant spacecraft. This work quantifies the design impacts for spacecraft willing to offer or use its data relay through the federation. Participation of spacecraft in a FSS ranges from sharing its unused resources (FSS operations on design margins) to partial or full spacecraft redesign reallocating part of mission functionality to the federated network (full FSS operations). Spacecraft in a FSS may act as suppliers of resources, customers of resources, or any hybrid role in the middle where satellites could be suppliers and customers of the network at the same time. This paper conducts a first design exploration of this tradespace assuming a dual supplier/customer nature of spacecraft (no hybrids), and assuming FSS operations on design margins. The paper develops a spacecraft model to study resource balancing in federations, including power, attitude control, communications and ISL subsystems. The goal is to quantify the necessary changes in the federated spacecraft in terms of system-level mass as a function of benefits obtained by participating in the federation. Impacts for small, medium and large spacecraft in Low Earth Orbit (LEO) offering a relay service on design margins are shown to be 12% (small platforms), 1.1% (medium platforms) and 0.6% (large platforms) in loaded mass. This results thus reveals that a relatively small mass overhead is required to participate in spacecraft federations to operate at design margins. Spacecraft relaying data through the federation may substitute space-to-ground link capabilities for FSS Inter-Satellite Links leading to mass savings up to 4.5% of loaded spacecraft mass. Finally, an analysis through lifetime shows that current power margins on spacecraft allow for a connection to the federation of 500s/orbit up to the 70% of spacecraft lifetime. The paper paves the way towards more sophisticated modeling of FSS operations aimed at addressing more general cases of participation schemes in FSS in hybrid/customer roles, and allowing designers to identify sweet spots in the level of participation to spacecraft federations.