Cambridge says that some exclamatives have interrogative form:
We sometimes make an exclamation using interrogative (question) word
order:
Have I got news for you! Peter and Michaela are getting divorced! (or,
less strong: I’ve got news for you!)
Did I do something stupid last night!
Other short exclamations can preceded it such as, Wow! Man! Gosh!.
CAGEL says about this:
Subject-auxiliary inversion is available as an option in exclamatives, though it is relatively infrequent and characteristic of fairly literary style. The effect of course is to make the structure more like an interrogative, so that from a grammatical point of view there will often be ambiguity as to clause type. (p. 920)
Now that we have established that such sentences are grammatically possible, we can look at the meaning you are trying to convey. Your sentence
Holy cow is it small!
by itself, cannot be understood as ironical, it is the context that will confer this connotation to it. You can say the same sentence about something tiny, in a different context. But in the right context, in front of a skyscraper, or a tall tree, or a mountain you need to climb, saying this sentence would definitely be ironic in a joking way, or it could express stupefaction.
As for your second sentence, you need an exclamation which will express a contradiction of something that has been said or beleived before. As I read it, I instantly thought of the informal expression My foot!:
used to mean that you do not believe what another person has just told you:
- "He says his car isn't working." "Not working my foot. He's just too lazy to come." (Cambridge)
So in the second case, I would say:
They are dead, my foot! ...
although the version suggested to you by @Lambie is also a good option:
Holy ..., are they ever dead!
