Each week Ross Rubin contributes Switched On, a column about consumer technology. If widespread rumors hold true, the coming weeks will see two of the most successful tablet vendors invade each other's screen size turf. Apple, which once rebuked the 7-inch tablet as unfit for normally proportioned human hands, appears ready to try its own hand at an iPad rumored to be on the larger side of the 7- to 8-inch range. Amazon, which lagged its bookselling rival Barnes & Noble in bringing out a color tablet, stands ready to introduce an inf
