Thanks to lameen (go check out his blog, Adela and Co., and I don't mean later!), my doubts concerning qaddāš as an exclamatory particle have now dispersed.
Side note 1: I called Silvia (who is a native speaker of Jordanian Arabic) and she informed me that in her native dialect, the proper way of saying "How big...! or "How pretty...!" would be Mā akbar...!, Mā aħsan...! etc. Using kam the way lameen suggested did sound acceptable to her Jordanian ears, but she would never use it that way.
Side note 2: The answer might have been right there before my eyes all the time. Lameen's comments reminded me of some phrases in Maltese where the particle kemm* (< كم) is also used in an exclamation. In search of examples, I picked up my copy of Olivier Friggieri's Fil-Parlament ma Jikbrux Fjuri and opened it at random. And lo! on page 81, I found the following: "Kemm tarahom koroh il-politiċi!" (lit. "How ugly you see them, the politicians!"). Two pages down, there it was again: "Kemm nixtieq li għandi l-pulptu tiegħek, Dun Benjamin!" (lit.: "How/How much I wish I had your pulpit, DB!"). Talk about serendipity...
A brief Google search uncovered several other examples, including this one: "Ara l-Ingliz kemm hu kreattiv!" ("Look at English, how creative it is!"). Great, thought I, 'tis probably some journalist trashing Maltese. I couldn't have been more wrong. Turns out this sentence was uttered by none other than Professor Manwel Mifsud in an interview with l-orizzont on the occasion of the founding session of the Kunsill Nazzjonali ta' l-Ilsien Malti**. Professor Mifsud has, as usual, a lot of interesting things to say about Maltese in general, its current state and future and he briefly comments on such things as English loan words and Maltese word formation. Read the whole thing.
* Maltese kemm turns out to be a rather interesting particle. Expect more on the subject later.
** I am not sure about the time line, since there is no exact date on the article. But since according to its website the Kunsill was founded in April 2005, I take that to be the date of the interview. And here's another one, this time in English.