Alpha particles are so small in size. Then how was Rutherford able to see them?

alpha particles are really smalll in size.
the observation was carried out because behind the gold foil, there was a screen coated with zinc sulphide ( ZnS).

when any alpha particle made it through the gold foil,  it made contact with the ZnS and it created a spark.

Rutherford was able to carry on with experiment by counting the number of sparks...
thumbs up!!
hope it helps :)