Once again, you need to go into the RTA. The people on the phones are half-wits and are only reading off the same internet page you're looking at.
They do have smaller plates, but as Chris mentioned above, they're personalised plates, not standard issue.
Head into your local RTA, and they'll have a section on the wall with every style of plate available attached to it. You'll be able to see the sizes in relation to each other. Ask them everything you need to know - style name, size, price, availability etc - and you're good to go. Easy peasy!
The on you'll be looking for is Black with white lettering. 2 letters and 3 numerals.