I don't know where to put the size argument, here I only managed to get a single edit size:
https://en.wikipedia.org/w/api.php?action=query&prop=revisions&rvprop=size&format=xml&titles=United_States_of_America
but I need the size of whole raw content as it is on last revision.
You are getting that, the page United States of America contains the following 69 bytes:
#REDIRECT [[United States]]
{{Redr|move|from long name|printworthy}}
What this code means is that it's a redirect and the name of the real article is United States.
This returns the size you want: 267582 bytes.
Another option would be let the API follow the redirect automatically using redirects
: