After posting, I noticed the deprecation warning for the package in favor of {spdv}, which you might take a look at in hope of better facilities and documentation.
I also failed to find much insight from the documentation and related search results. ColorBar() function is supposed to allow fine-grained control of the legend, but there is nothing I saw to show out to save the PlotStereoMap() output as anything but an exported output file and ColorBar() doesn't even have that option. I couldn't get this to work with {grid} and {gridExtra}.
I'd suggest printing the map with
drawleg = FALSE,
fileout = "the_map.tiff"
and reproduce the legend in ggplot2, then save in like format and use a photoeditor to stictch them together and adjust relative size to preference.
library(grid)
library(gridExtra)
library(s2dverification)
#> Loading required package: maps
#> Warning: This package has been deprecated. Please consider using the package 's2dv' instead.
#>
#> Attaching package: 's2dverification'
#> The following object is masked from 'package:base':
#>
#> Filter
Data <- matrix(rnorm(100 * 50), 100, 50)
x <- seq(from = 0, to = 360, length.out = 100)
y <- seq(from = -90, to = 90, length.out = 50)
cols <- c("dodgerblue4", "forestgreen", "yellowgreen",
"white", "yellow", "orange", "red", "saddlebrown")
the_map <- PlotStereoMap(Data, x, y, latlims = c(60, 90),
toptitle = "This is the title",
drawleg = FALSE)#,

# fileout = "the_map.tiff")
the_legend <- ColorBar(brks = seq(-4,4,1),
cols,
vertical = FALSE,
plot = TRUE)
