Parsing the value and copying it to another column in postgresql
If your data is consistent, so that after a number it always contains a space and then something else, then the following expression will get you the number part:

split_part(original_size, ' ', 1) 


This will split up your string along the spaces with the remaining chunks are indexed from 1 up to the last one, then the first one is picked.  There are, of course, other possibilities around, for example using a regex, which probably other will add later :)

Now you can construct your UPDATE statement:

UPDATE size
   SET processed_size = split_part(original_size, ' ', 1);


Notice that there is no WHERE clause as you want to set the second column for every row in the table.

You find an example on SQLFiddle.