您可以添加一些LaTeX代码和escape(escape=f)以在脚注上实现更小的字体大小。

一个可重复的例子：

---
title: "Footnote"
author: "bttomio"
output: pdf_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE, message = FALSE, warning = FALSE)
```

```{r evensmallerfootnote}
library(tidyverse)
library(kableExtra)

data <- structure(list(Category = c("Internally Managed", "Externally Managed", 
"Asset Swaps", "Total"), Percent = c("44.1%", "32.5%", "23.4%", 
"100%")), row.names = c(NA, -4L), class = c("tbl_df", "tbl", 
"data.frame"))

data %>%
 kable(format='latex',booktabs=TRUE, align = 'lr',
        caption="Breakdown \\label{Table3}") %>% 
  kable_styling(latex_options = c("HOLD_position", full_width = T), font_size = 9) %>% 
  row_spec(0,bold=T,color = 'white', background = '#7c3042') %>% 
  row_spec(1:4, hline_after = T) %>% 
  row_spec(4, bold = T) %>% 
  column_spec(1:2, width = '5cm') %>% 
  footnote(general = "\\\\tiny{*This is what I want my footnote to be}", threeparttable = TRUE, escape = F)
```


```{r smallerfootnote}
library(tidyverse)
library(kableExtra)

data <- structure(list(Category = c("Internally Managed", "Externally Managed", 
"Asset Swaps", "Total"), Percent = c("44.1%", "32.5%", "23.4%", 
"100%")), row.names = c(NA, -4L), class = c("tbl_df", "tbl", 
"data.frame"))

data %>%
 kable(format='latex',booktabs=TRUE, align = 'lr',
        caption="Breakdown \\label{Table3}") %>% 
  kable_styling(latex_options = c("HOLD_position", full_width = T), font_size = 9) %>% 
  row_spec(0,bold=T,color = 'white', background = '#7c3042') %>% 
  row_spec(1:4, hline_after = T) %>% 
  row_spec(4, bold = T) %>% 
  column_spec(1:2, width = '5cm') %>% 
  footnote(general = "\\\\footnotesize{*This is what I want my footnote to be}", threeparttable = TRUE, escape = F)
```

```{r footnote}
library(tidyverse)
library(kableExtra)

data <- structure(list(Category = c("Internally Managed", "Externally Managed", 
"Asset Swaps", "Total"), Percent = c("44.1%", "32.5%", "23.4%", 
"100%")), row.names = c(NA, -4L), class = c("tbl_df", "tbl", 
"data.frame"))

data %>%
 kable(format='latex',booktabs=TRUE, align = 'lr',
        caption="Breakdown \\label{Table3}") %>% 
  kable_styling(latex_options = c("HOLD_position", full_width = T), font_size = 9) %>% 
  row_spec(0,bold=T,color = 'white', background = '#7c3042') %>% 
  row_spec(1:4, hline_after = T) %>% 
  row_spec(4, bold = T) %>% 
  column_spec(1:2, width = '5cm') %>% 
  footnote(general = "*This is what I want my footnote to be", threeparttable = TRUE, escape = F)
```

-输出

您可以遵循以下操作：