I fell off bicycle on sat. and have a purple bruise that is a 6 x 10 inches. can i safely resume riding on a trainer/stationary bike at this time ?

Yes, ....: Apparently, you got ecchymosis (bruise) from bleeding by severe contusion, which may slowly extend and expand to a wide degree and span over 3-5 days, and will spontaneously resolve over 2-8 weeks leaving almost no residual mark but not always. As to when to resume exercise, you may do so as long as being tolerable & no undue swelling. More? Seek evaluation and counseling.