What is the suitable size for a customized Articulate template?

Maybe the trick to solve this is to adjust the slide design a little. Website are often laid out in a grid system with more space in the margins so the eyes don't have to track along long lines of text. If users are finding it hard to follow along text that is full width try breaking it into columns - a little like a newspaper. That will be easier especially for people who tend to skim down the centre of a column and pick up the edges with peripheral vision. 

Another readability idea is to use 1.5 line spacing so the eye can follow a single line more effectively, without being pulled down to the next.

I haven't seen any specific research into screen width, but some testing on a few design variations may help you come up with your own "standard". Or, if you take a look at the WCAG guidelines for accessibility that might point you in a direction that works for you.