library(terra)
library(RColorBrewer)

# sample polygon
p <- system.file("ex/lux.shp", package="terra")
p <- terra::vect(p)

# sample raster
r <- system.file("ex/elev.tif", package="terra")
r <- terra::rast(r)
r <- terra::crop(r, p , snap = "out", mask = T)

terra::plot(r,
           col = brewer.pal(9, "pastel1"),
           cex.main = 2,
           smooth = T,
           legend = T,
           plg = list(title = "Score"),
           axes = TRUE,
           mar=c(3,3,3,6))
plot(p, add = T)

How do I change the size and orientation of the legend title 'Score'.
I want to orient the title so that it is vertical and follows along the
legend and also change the size of the legend title?

change the size and orientation of legend title while plotting raster
You can add text whereever you want. For example
library(terra)
p <- terra::vect(system.file("ex/lux.shp", package="terra"))
r <- terra::rast(system.file("ex/elev.tif", package="terra"))

plot(r, mar=c(3,3,3,6))
text(x=6.77, y=49.95, "Score", srt=-90, cex=2, xpd=NA, pos=4)
lines(p)


