How to measure the area of a hexagonal bin generated using ggplot2?

I've generated a number of plots using the geom_hex and stat_summary_hex functions in ggplot2, and I've adjusted the number of bins until the plot "looked right." That said, I'd like to know the area each bin represents to help interpret the plot.

Below is a simplified example of the set of example data and plot:

library(dplyr)
library(ggplot2)
library(rgeos)
library(maptools)
library(rgdal)
library(hexbin)

crime <-read.csv("https://opendata.arcgis.com/datasets/6eaf3e9713de44d3aa103622d51053b5_9.csv", stringsAsFactors = F)

homicides <- crime %>% dplyr::select(long = ï..X, lat = Y, OFFENSE) %>%
filter(OFFENSE == "HOMICIDE")

coordinates(homicides) <- ~long + lat
homicides.df <- data.frame(homicides)

homicides.hex <- ggplot() +
geom_hex(data = homicides.df, aes(x=long, y =lat),
fun = sum, bins = 20) +
coord_equal()
print(homicides.hex)

Is there a function or method to measure the area (in square miles or kilometers) of an individual hexagon when the x and y axes are latitude and longitude? • Each row of cells has different area than the row above or below it. – Vince May 27 '17 at 0:42
• @Vince I'm actually interested in measuring the area of a single hexagon that was generated. I edited the question to clarify my need. – Dave May 27 '17 at 4:54
• A polygon is a polygon, no matter how many sides. All you need is a geodetic area function. – Vince May 27 '17 at 11:53