Too small / large
Sorry chaps, but according to the Airbus/Boeing websites there's not much of a difference between them (note, Airbus doesn't list any data on the A350, so I've chosen those for the A332):
7E7-8: Cross section 5.74m
A332: Cross section 5.28m
7E7-8: Length 59m
A332: Length 57m
Boeing states 217 seats for the -8, I haven't seen what Airbus promises but their data from the -200 seems to indicate around 250 seats. However, if the stats from Boeing are for an aircraft kitted out for ULR and the -200 'just' for LR, then that might explain the difference. Remember, SQ only have around 180 seats in their A345s doing the ULR services to the states.
In other words, for all intents and purposes they're roughly the same size.
But, as POL says, at this point in time both are but paper aeroplanes - the A350 to a slightly larger extent than the 7E7. Paper is a very useful tool; one can prove almost anything on paper. Let's see them build, and flown, and then pass judgement.