I always assumed the Klingons were initially probed by V'ger prior to the start of the film, so as Klingons their first instinct was to attack. Size matters not to Klingons (and Yoda). The actual calculation of the size of V'ger is not something I ever cared to analyze. It was clear the writers were trying to say this thing was bigger than anything ever imagined, and that was good enough for me.
I know this isn't quite on topic, but I think it would have been far more interesting if the crew was seeing memories ("data") of planets, aliens and objects on the screen as they initially passed through on the way to the center/heart of v'ger -- the sort of stuff hinted at during Spock's walk, but more of it and earlier in the film. So much time passes and we're just seeing reaction shots cutting back and forth to beautiful but random animations. That should be the most jaw-dropping part of the film, but it's the point I usually fall asleep. But even if it were scripted that way, I'm sure it would have been impossible given the level of effects back then and the budget/time concerns. They were lucky to get those animations I suppose. But it would have been more interesting to watch, IMO. That, and changing the uniforms.