Welcome to JREF!
1. Japanese giant hornets
There's no need to worry about it unless you're going into the mountains and waking through bushes.
2. Sakura (Cherry blossoms)
I cannot think of big festivals related to sakura, but if you go to any parks famous for sakura, you'll see a lot of people having picnic under cherry trees.
In some place they put up lanterns so you can enjoy sakura after dark.
In some places you'll find many stalls selling drinks and food and souvenirs when cherry trees are in bloom. It is not exactly a festival but I'm sure you'll enjoy the festive atmosphere.
3.
I've picked Spring to travel because i'd really like to see the cherry blossoms in full bloom across the country - are there any amazing festivals that i must go to in particular during this time?
3. Is there by any chance a translated website (English) saying which Japanese music bands/artists are touring next Spring in Japan? I'd love to go and see a few if possible while i'm there.
4. Clothes size
http://www.usatourist.com/english/traveltips/shopping/shopping-womens-sizes.html
Women of 6' (= 180cm) tall is very tall in Japan.
You might want to got department stores where they have special corner called "size shop". They usually have clothing for tall women.
(Isetan department store in Shinjuku has rather a wide range of clothing of special size.)
5. Places of interest
It really depends on what kind of things you want to see in Japan, and the area you'll be travelling.
How many days are you going to stay, which are of Japan are you travelling?
What kind of things you prefer, something old and traditional? Modern and high-tech? etc.