I get 1000 roaches for about 50 bucks shipped to my door in 2 to 3 days from delicious dubias. I'd guess at 15 inches mediums to large would do. Each vendors sizes are slightly different so check how big they say their sizes are. The roaches should be about the size of the space between the eyes of your beardie. An average medium dubia of about 3/4 of an inch will easily be as large as 3 or 4 crickets plus it being more nutritional, I'd say 5 crickets per roach is similar is nutrition. As for lifespan and ease of care there isn't much easier then a dubia. They seems to last at least a couple weeks without food or water, give then food (dog food and left over beardie greens) and they will live on for a very very long time and will start to breed once they molt into adults. I put about 10 to 15 adult dubias in my bioactive tegu enclosure 3 years ago, there are still breeding roaches in their today and the tegus constantly hunt them. Most of the people that sale dubias also have a selection of other insects like nutrigrubs and hornworms, its always a good idea to get a couple different insects to go with your dubias. Dubias are the staple insects so they should be eating more dubia then any other bugs but variety is the key to a happy life.