How to choose one row out of several of equal value in a column, depending on the highest value of another column?

Question

So I have a data frame such as this, containing species names, IDs, markers, and size of DNA sequences:

 Species           |  ID| marker| size   
-----------------------------------------------------
Tilapia guineensis |   1|   co1 |  400
Tilapia guineensis |   1|   co2 |  300
Tilapia guineensis |   1|   co2 |  700
Tilapia guineensis |   1|   co2 |  900
Tilapia zillii     |   2|   co5 |  600
Tilapia zillii     |   2|   coi8|  200
Tilapia zillii     |   2|   coi8|  500
Eutrigla gurnardus |   5|   co1 |  100
Eutrigla gurnardus |   5|   co2 |  200 
Sprattus sprattus  |   6|   co3 |  300 
Sprattus sprattus  |   6|   co4 |  400
Sardinia pichardus |   7|   co1 |  800
Sardinia pichardus |   7|   co2 |  800

I would like to keep only one row for each ID, but I want to keep the row that has the highest value in the size column, regardless of the species and marker columns. If the rows have equal ID and equal size, I would like to just keep one randomly. My output would be:

 Species           |  ID| marker| size   
-----------------------------------------------------
Tilapia guineensis |   1|   co2 |  900
Tilapia zillii     |   2|   co5 |  600
Eutrigla gurnardus |   5|   co2 |  200 
Sprattus sprattus  |   6|   co4 |  400
Sardinia pichardus |   7|   co2 |  800

Answer 1

A possible solution:

library(dplyr)

df %>% 
  group_by(ID) %>% 
  slice_max(size)

#> # A tibble: 4 × 4
#> # Groups:   ID [4]
#>   Species               ID marker  size
#>   <chr>              <int> <chr>  <int>
#> 1 Tilapia guineensis     1 co2      900
#> 2 Tilapia zillii         2 co5      600
#> 3 Eutrigla gurnardus     5 co2      200
#> 4 Sprattus sprattus      6 co4      400

Answer 2

data.table option:

library(data.table)
setDT(df)[, .SD[which.max(size)], by=ID]