Grouping data in dataframe

I have a dataframe (obtained from read.table()) which looks like

     ExpA   ExpB   ExpC   Size
1      12     23    33      1
2      12     24    29      1
3      10     22    34      1
4      25     50    60      2
5      24     53    62      2
6      21     49    61      2

now I want to take all rows that have the same value in the "Size"
column and apply a function to the columns of these rows (for example
median()). The result should be a new dataframe with the medians of the
groups, like this: