I am using a 4 hole vertical line entrance on the side next to the end as my own experiment.
I am building and using ktbh's with a sloped side.
the top hole in the line is 3/8" followed by a 1/2" hole beneath it, a 3/4" hole beneath that and a 1" hole as the last hole on the bottom .
My intent on the 4 vertical hole entrance is that the 3/8" hole at the top is the primary and 'safety' entrance. it allows the condensation to escape in winter and provides a single, small and very easily defended entrance in times of robbing or small colony thus fewer bees to defend..
The lower hole are primarily for allowing greater traffic, weather and circumstances permitting, and can be easily covered over to reduce down to the top 3/8" hole only.
The nature of the ktbh having slanted sides provides some bit of extra weather protection for the entrance holes.
I like to have the holes placed approx 2 to 4" from the end board which pretty much accomplishes having an end entrance at the same time and the bees work toward the opposite end board as they build comb.
This can make use of one to two follower boards (one at the front and one at the rear, if one wants to use them) to keep space and inner temperature more manageable by the bees until they fill the entire hive. The follower boards can also then be easily removed for inspections of the end combs without necessarily greatly disrupting or alarming the bees.
Getting back to the issue of top bar sizes, I originally started out using the 2 size bars as Micheal mentions, but am now implementing 1 3/8" wide top bars universally as I am finding that works better in my efforts.
My advice is to read all you can find and use your own educated judgment to decide what will work best for you, the bees and goals you have in beekeeping. What works best for me or Micheal or anyone here here may not be the best for anyone else for dozens of reasons dealing with location, intent, personal philosophy, etc...
Enjoy the bees, have fun,