I think that the issue is really more about what works, musically speaking, than it is about what the bells symbolize, or don't symbolize. Mahler was a thorough enough composer that if he had wanted particularly small bells - or just a few bells - he would specified that in the score. As it is, the score simply says Schellen.
The Concertgebouw, for example, uses a nice, deep sounding set of sleigh bells. Yet, the Concertgebouw has a direct connection to Mahler's 4th symphony: there was that famous concert where Mengelberg conducted the fourth symphony on the first half of the concert, then Mahler himself conducted it on the second half. If there had been any controversy over the bells, Mahler would have straightened the issue out, right there and then. As for the imagery in one's mind, can't that be left up to the listener? Remember, Mahler dropped the whole notion of program notes, simply because so many people were taking things too literally, or taking them clear out of context.