I have recently crossed over from LV, so I can totally relate...
I think the Box is around the same size and shape as a Speedy, it's 11.5x6.8x5, and a Speedy 25 is 10x7x6. The Twiggy is a little longer.
I just got my first Bbag last week and it's a City, which is an excellent size. As meemie said upthread, I expected it to be a little too big, but it isn't at all. I highly recommend it for your first Balenciaga, if you carry more than just essentials. The most remarkable thing (other than the soft leather and color, hehehe) is its light weight - even full of stuff, it doesn't weigh anything! I don't know how they do that!
Be careful...I just ordered my second City, and I already have my eye on a third! Balenciaga is easily as addicting as LV, and just as expensive.