您可以在geom_label中更改size，如下所示：

library(ggalluvial)
library(ggplot2)
library(dplyr)

df <- data.frame(status = c("open", "close", "close", "open/close", "close"), 
                 stock = c("google", "amazon", "amazon", "yahoo", "amazon"), 
                 newspaper = c("times", "newyork", "london", "times", "times"))

# Count the number of occurance for each alluvial
df <- df %>% dplyr::group_by(stock, newspaper, status) %>% 
  summarise(n = n()) 
#> `summarise()` has grouped output by 'stock', 'newspaper'. You can override
#> using the `.groups` argument.

# Define the factors
df$status <- factor(df$status, levels = c("open", "open/close", "close"))
df$stock <- factor(df$stock, levels = c("google", "amazon", "yahoo"))
df$newspaper <- factor(df$newspaper, levels = c("times", "newyork", "london"))

# Plot the alluvial as in https://cran.r-project.org/web/packages/ggalluvial/vignettes/ggalluvial.html#alluvia-wide-format
ggplot2::ggplot(df, aes(y = n, axis1 = stock, axis2 = newspaper)) +
  ggalluvial::geom_alluvium(aes(fill = status), width = 1/12) +
  ggalluvial::geom_stratum(width = 1/12, fill = "black", color = "grey") +
  ggplot2::geom_label(stat = "stratum", aes(label = after_stat(stratum)), size = 2) +
  ggplot2::scale_x_discrete(limits = c("stock", "newspaper"), expand = c(.05, .05)) +
  ggplot2::scale_fill_brewer(type = "qual", palette = "Set1") +
  ggplot2::ggtitle("Alluvial-Test")
#> Warning: Using the `size` aesthetic in this geom was deprecated in ggplot2 3.4.0.
#> ℹ Please use `linewidth` in the `default_aes` field and elsewhere instead.

创建于2022年12月11日，使用reprex v2.0.2

这至少是一个已知的问题。并且有几个here的解决方案。我个人喜欢ggfittext的解决方案。下面是我尝试解决您的问题的方法。请注意，@Quinten的解决方案也非常好：
我们可以用alpha=...来拟合黑色：

#install.packages("ggfittext")
library(ggfittext)
library(ggalluvial)
library(ggplot2)
library(dplyr)
# Plot the alluvial as in https://cran.r-project.org/web/packages/ggalluvial/vignettes/ggalluvial.html#alluvia-wide-format
ggplot2::ggplot(df, aes(y = n, axis1 = stock, axis2 = newspaper, label=after_stat(stratum))) +
  ggalluvial::geom_alluvium(aes(fill = status), width = 1/12) +
  geom_stratum(width = 1/12, fill="black", color="grey") +
  ggfittext::geom_fit_text(stat = "stratum", width = 1/12, min.size = 3, color="grey") +
  ggplot2::scale_x_discrete(limits = c("stock", "newspaper"), expand = c(.05, .05)) +
  ggplot2::scale_fill_brewer(type = "qual", palette = "Set1") +
  ggplot2::ggtitle("Alluvial-Test")

我认为geom_text旋转90度会得到更好的结果：

ggplot(df, aes(y = n, axis1 = stock, axis2 = newspaper)) +
  geom_alluvium(aes(fill = status), width = 1/12, color = "white", size = 2) +
  geom_stratum(width = 1/12, fill = "gray30", color = "white", size = 2) +
  geom_text(stat = "stratum", aes(label = after_stat(stratum), angle = 90),
            color = "white", size = 8) +
  scale_x_discrete(limits = c("stock", "newspaper"), expand = c(.05, .05)) +
  scale_fill_brewer(type = "qual", palette = "Set1") +
  ggtitle("Alluvial-Test") +
  theme_void(base_size = 20) +
  theme(plot.margin = margin(20, 20, 20, 20))