# x is already a factor 
levels(x) 
# [1] "Large" "Medium" "Small" 

# various ways to reorder levels to be Small, Medium, Large 
factor(x, levels = levels(x)[3:1]) 
factor(x, levels = c("Small", "Medium", "Large")) 

# x is not yet a factor (e.g., a character vector or data.frame col) 
factor(x, levels = levels(factor(x))[3:1]) 
factor(x, levels = c("Small", "Medium", "Large"))

Reorder levels of a factor

R