As the person managing the Boost Nomination Pilot program, I'm here to say that story length is NOT a factor in a story getting selected for Boost. I've seen 1 minute reads Boosted (poetry!) and 25 minute reads Boosted (thoughtful product documentation).

Here's a relevant quote from the Medium Quality Guidelines:

Is the story well-crafted? The story is well-written, free of errors, appropriately sourced, and narratively strong. The content, editorial choices, and central point of the post are clear and compelling. The length of the story is appropriate and serves the purpose and context of the story itself (well-crafted stories can be short or long).

Read more here:

https://help.medium.com/hc/en-us/articles/360006362473-Medium-s-Quality-Guidelines-How-real-humans-review-stories-for-Boost