This is a total, wild guess, but I would expect the download to be at least 1G (and quite likely more). GalCiv II (the base game) is 1.4GB as reported by Impulse, and I expect that Elemental will have higher amounts of detailed art assets.

As for the other part of your question, you might want to just email sales@stardock.com - they should definitely be able to tell you and I've never had a problem dealing with them. Of course, they won't respond 'til Tuesday or Wednesday I'm sure (Monday is a US holiday most places).

Its difficult to say exactly without feedback from the devs. Considering that this game will be in a state of development, whatever size it starts out with will likely grow.

Fortunately Impulse doesn't tend to download the whole game again when updating. It'll will only update the components that have changed since last time. So long as you keep up to date with the downloads, any updates will likely be small. Odd are, only the first download will be the big one.

That said, you might want to be ready for a large download. If I recall correctly, Demigod started around 1 Gb, but later grow to nearly 3 Gb by release.

This actually sounds like an Impulse question to me, because that's the app we'll be required to use to download and install the betas and released game.

But I'm one of those Stardock geezers who still has a few fingers on his cold, dead hands wrapped tightly around Stardock Central, which means I have no idea at all if or how Impulse would handle interruptions during a basic download-install  process (I'm working on this 'attitude problem,' but progress is slow).

My hope (alright, my expectation, but I'm an awfully demanding user...) is that Impulse would be able to resume an interrupted download on demand, whether or not download limits were the reason for the interruption.