For Echinopsis hybrids its diameter size that determine if they are big enough to bloom. Depending on the cross if smaller or larger growth type but most on average begin flowering at 3" and larger in diameter. Most Trichocereus hybrids need 12"+ in hieght on average, also depends on perticular variety.

Most likely the growers you mention that they did not bloom for them after several seasons; thats most likely due to they probably never fertilized regularily during the growing season or have not even repotted them with fresh soil mix ever. Echinopsis are heavy feeders and if given food-fertilizer or fresh soil mix, they will bloom over and over during the warm season. You can see for yourself starting Feb usually in full bloom though from March thru October at the nursery as we are open most weekends Epicacti.com