Dictionary is the application, which shall be adjusted to the position {0, 20} and to the size {1280, 800}.
Note that this is the size of the plain window without a shadow. If you do a screenshot, you can omit a shadow by holding the ⌥ [option] key.
tell application "System Events"
set ssProcess to first process whose name is "Dictionary"
tell ssProcess
tell first window
set position to {0, 20}
set size to {1280, 800}
end tell
end tell
end tell
tell application "Dictionary" to activate