Is the reply fine?

I would probably say "two or three ears". The response you suggest is OK, but sounds a bit awkward to my ears.

two to three implies that size matters, give me two if they are large, three if they are small. Two or three implies that I don't care about being more specific, either two or three would be fine.

http://wiki.answers.com/Q/How_much_corn_is_on_one_ear_Specifically_what_is_the_portion_one_cup_or_one_half_cup_etc