I don't see what they hope to accomplish with such a small force. Ok, so they might be able to capture an island or two, or establish a beachhead, but then what? It's not likely going to open the Strait of Hormuz.
Neocon Think Tank documents, like
Which Path to Persia, from 2009, about an invasion of Iran talk about 250 000 soldiers, and I'm not sure if that would be enough.
Iran also successfully fought off the invasion from Iraq in 1980, consisting of some 200 000 soldiers, in a war that lasted 8 years, with millions of casualties.