There are a number of ways to do this, depending on your data and use case. I particularly like gghighlight -- this vignette describes the many options it has. Here are a few examples:
library(tidyverse)
set.seed(1)
df <- tibble(
time = rep(1:5, 3),
class = rep(letters[1:3], 5),
value = rnorm(15)
)
# set size of lines manually
df %>%
ggplot(aes(time, value, color = class)) +
geom_line(aes(size = class)) +
scale_size_manual(values = c(a = 3, b = 0.5, c = 0.5))

# draw wider line separately from others and set size
df %>%
filter(class != "a") %>%
ggplot(aes(time, value, color = class)) +
geom_line() +
geom_line(data = filter(df, class == "a"), size = 3)

# use gghighlight()
df %>%
ggplot(aes(time, value, color = class)) +
geom_line() +
gghighlight::gghighlight(class == "a")

Created on 2020-06-01 by the reprex package (v0.3.0)