library(dplyr)
dat <- read.table(text =
        " Animal Color Size
          Cat Orange 10
          Dog Black 20", header=TRUE)

Animal  Color   Size
Cat     NA      NA
Cat     Orange  NA
Cat     Orange  10
Dog     NA      NA
Dog     Black   NA
Dog     Black   20

expand.grid(dat$Animal, dat$Color, dat$Size) %>% arrange(Var1, Var2, Var3) #Note: this does not give the correct answer.

dat <- c("Cat", "Orange", 10)

counter <- 1
datInner <- list()
for(i in 1:length(dat)){ # loops through 3x

  # i <- 3
   datInner[[i]] <- dat[1:i]
   counter <- counter + 1

}

library(plyr)
# Adapted from http://stackoverflow.com/questions/17308551/do-callrbind-list-for-uneven-number-of-column
plyr::rbind.fill(lapply(datInner, function(y){as.data.frame(t(y),
            stringsAsFactors = FALSE)}))

    # V1     V2   V3
    # 1 Cat   <NA> <NA>
    # 2 Cat Orange <NA>
    # 3 Cat Orange   10