If you want to feed them a cicada I'd try to find a small cicada, around maybe between 0.5-1 inch? Closer to 0.5 would be better. I dont know if they even emerge at that size, what species of cicada we're talking about, etc. I dont even know much about cicadas. Just for the record you do run the risk of transmitting parasites from wild insects to your frogs. Admittedly this is not the end of the world, but you'll probably have to deworm them eventually if you feed them wild insects. I feed my Woodhouse's toads wild bugs sometimes(grasshoppers, and some pillbugs) because I caught them in the wild. They really love termites and crane flies(mosquito hawks etc. Lots of names).