Well, I can replicate the Previewing issue using RStudio Cloud (RStudio 1.2.451.1) at least. With these file contents:
---
title: "Sample"
output:
html_notebook:
df_print: paged
---
```{r}
library("ggplot2")
targetCustomers <- data.frame(customer = c('cust1'), value = c(19))
chartHeight <- 4
```
```{r, fig.height=4, fig.width=4}
ggplot(targetCustomers) +
geom_col(aes(x = customer, y = value), fill = "purple") +
coord_flip()
```
```{r, fig.height=chartHeight, fig.width=4}
ggplot(targetCustomers) +
geom_col(aes(x = customer, y = value), fill = "purple") +
coord_flip()
```
If I run all chunks, and then Preview, I get this:
http://rpubs.com/jcblum/399125
Note that the two plots should have identical sizes/aspect ratios, but they do not. The two plots have different size/aspect ratios in the inline preview, as well.
Knitting to HTML works fine, as previously noted (the two plots have the same size/aspect ratio).
(Edited because I initially copy-pasted the source after I'd tried knitting, so of course a new output line had been added to the YAML which was not there when I initially Previewed)