Just a thought... is it possible to make a weapon cause a planet to disappear from the game?
Actually, in the show, Annorax was removing the inhabitants from the timeline, more often than not. Usually, this would result in a different, more wild appearance from space. A simple texture change weapon, with accompanying sensor readings (if any) changed would suffice.
As for a size comparison, if I recall correctly from that Hirogen/Nazi/Holodeck episode, Voyager was surrounded by four Hirogen Hunter vessels, and they looked slightly larger than Voyager. That said, I don't think the size was very consistent, as I think I remember a shuttlecraft looking larger than expected next to one in one of the early Hirogen episodes (you know, when they were still tall)...