The initial load does have impact as it tries to sync all data quickly. Sizing with XDCR has a few parameters based on document size, mutation rate etc and assumes initial sync is something you can do at a low traffic period so won’t give you a precise answer. However there is some guidance here for you;
http://blog.couchbase.com/how-many-nodes-part-2-sizing-couchbase-server-20-cluster
thanks
-cihan