Altering flextable width for use in RMarkdown Tufte package

Question

I often use the Tufte handout in RMarkdown output to pdf to write technical documents. I have one that will have a lot of regression results, so I'm attempting to use flextable for pretty printing the results. However, I can't find any way to get flextable results to respect columns or widths. Here is my minimal example of a doc w/ a flextable that doesn't work followed by a graph that fits where it should:

    ---
    title: "A Tufte Handout Example"
    output:
      tufte::tufte_handout:
      citation_package: natbib
        latex_engine: xelatex
    bibliography: skeleton.bib
    link-citations: yes
    ---

    ```{r setup, include=FALSE}
    library(tufte)
    options(htmltools.dir.version = FALSE)
    ```

    ## Margin Figure
    ```{r flextable1, echo=F, fig.margin=T}
    library(flextable)
    ft <- flextable(head(airquality))
    ft <- autofit(ft) 
    theme_box(ft)
    ```

    ## It should be over there...
    ```{r fig-margin, fig.margin = TRUE, fig.cap = "MPG vs horsepower, colored by             transmission.", fig.width=3.5, fig.height=3.5, cache=TRUE, message=FALSE}
    library(ggplot2)
    mtcars2 <- mtcars
    mtcars2$am <- factor(
      mtcars$am, labels = c('automatic', 'manual')
    )
    ggplot(mtcars2, aes(hp, mpg, color = am)) +
      geom_point() + geom_smooth() +
      theme(legend.position = 'bottom')
    ```