-Generally 10-15% of body weight or up to 1.5 times the thickest part of the snake are rules of thumb for people who don't know how to figure out proper food sizes. Never go by size of the head...some people have and it's not reliable of course. Also, snakes can eat themselves to illness or death so it's best to follow this rule of thumb so that doesn't happen. Single prey items offered at once is usually regarded as the best way to go.
-A lot of MBK do fine on an adult mouse or small rat for food items. What you offer will depend on the adult size and weight of your individual animal as adult sizes vary.
-Feeder lizards often have internal parasites that can be detrimental to your snake so it's something I would advise to stay away from. Variety is not necessarily a bad thing, but rodent eating snakes do very well on a diet consisting of whole rodents without the need for variety. Chicks, quail, as well as quail eggs are all acceptable alternatives. Chicks would be appropriately sized for adults.
I'm sure others will chime in with their own opinions as well and they may differ a bit from mine, so take the opinions and combine it with a bit of your own personal research to find out what will work best for you.