Sound to me like that's a variation of the Zeno argument that you cannot move or walk across the room because there are infinite steps in between.
Someone else ( Joshua ) probably can state the response better than me, but Epicurus rejects the argument that matter is infinitely divisible so as to make motion impossible, and that presumably would apply to this question as well.
As to conceptualizing infinity that's an excellent question too. I presume part of the answer there is that it would be more difficult to conceptualize an END to space or number of atoms than it would be to conceptualize unlimited amounts of both. That's the argument that is stated at length in Lucretius Book One at 968 in more detail (including the javvelin argument) than is included in the letter the Herodotus.
Again as with your other question I think you're touching on something where we have at least some relevant information in Philodemus' "On Signs," this time under the heading of "inconceivability."