Question: If an educational company described their session methodology as "high in reach" does it mean:

1. the size of the audience
2. the effectiveness of the training
3. other?

The original sentence in a press release was just "Our trainings are low in cost, high in reach".

I see that this comparison with cost is sometimes being used together [for example](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2957672/) "*Our interventions must be high in reach but low in cost in order to most persuasively demonstrate worth in intervention*"

Answer: 
"High in reach" is a non-idiomatic way of saying that it *reaches* many people. This uses the word *reach* in the following sense:

> 
> **reach** *noun*
>  2 The extent or range of something's application, effect, or influence.
>  ‘he told a story to illustrate the reach of his fame’
>  [- ODO](https://en.oxforddictionaries.com/definition/reach)
> 
> 
> 

Consider the fuller quote of the [link you provided](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2957672/) (emphasis, mine):

> 
> When all is said, the promise of the history of diffusion scholarship and diffusion practice is a promise of efficiency in intervention: **Communicating** an innovation **to a special small subset** of potential adopters so that they, in turn, will **influence the vast majority of other potential adopters** to attend to, consider, adopt, implement, and maintain the use of worthy innovations. Our interventions must be high in reach but low in cost in order to most persuasively demonstrate worth in intervention.
> 
> 
> 

It's talking about accessing a small group that would *reach* the larger (target) audience.
"High in reach" in this context would mean the goals are high but within reach. It's a motivational phrase that you have high and lofty goals but with your abilities they are well within reach, or that you are well within your ability to meet those goals.