The most common monitor in 1979 was the Sanyo model VM4209 monochrome 9" monitor pictured above. There were no "computer" monitors on the market, and the little Sanyo security monitor was the least expensive and a perfect match for a Disk][ atop your Apple][. Mine pictured here:

http://web.me.com/henryspragens/stuff/Photos/Pages/Computers.html#0

In 1980, Sanyo came out with a 15" color monitor which the well-equipped Apple user could buy instead of using a TV with an RF modulator. Those Sanyo color monitors appear all over the sets of Blade Runner, if you want to know what one looks like. The most common screen for early Apples was a TV, though. If you're going for "period correct", look for a 1979 vintage Sony Trinitron or equivalent.