eronika życzy ci dużego zdrowia (...)?
agree that it is because she is wishing 'much health'.
there must be some uncountable aspect here. some less-specific-in-terms-of-amount, more-meaningful-in-terms-of-quality thing going on here. (forgive me - i am an instinctual learner now - and have no excuse to offer for that, given my education, except that it feels good)