3 Victory SDs easy, or a number of smaller ships. When we get larger capitals I'm assuming people will move to larger points games, but we don't even have the starter box so it's all speculation on how things will be costed. If the Imperial(or Tector, or whatever) SD ends up being 175-200, then it does limit you to 2 ships ish, at which point the Empire player is playing a skirmish game instead of a grand fleet battle. The big Mon Cal's will likely do similar things to rebel 300 pt lists, but then again, we don't know enough about the game to do more than speculate.
From my relatively soft education on naval matters, I'd say 2-6 ships is a "squadron"(though 3-4 of generally only a single class of ship in a squadron IIRC) or "flotilla" rather than a "fleet". It does rather look like the standard 300 point limit really won't be epic fleet scale combat I'm afraid.
Even in the simplified format we're looking at so far a full on fleet battle would take several hours to play out.
That having been said, as in epic X-wing, whipping out your big, impressive ships will likely require a larger points limit to be set, which may lead to full on fleet scale engagements, much larger points limits to be set, and much longer playtimes.
Those of us with limited funds and carefully managed finances will have to work our way up to fleet scale engagements slowly.
We haven't been shown Ion cannon that I know of, but I'd be flatly surprised if we didn't get them as an upgrade card. The demo games have been keeping it simple to expose as many people to the core mechanics as possible, which I approve of in a general sense.