Hello White_Rose!
Welcome to the boards! {thumb2}
You understood everything pretty good!

Just a few comments from me:
Would it be possible to use smaller envelops?
Of course, the envelopes really don't have to be that big, although a lot of people sends this size. However, I almost always send my letters and other things in A5 envelope (5x7 inches I guess) and my SASE (smaller envelopes) are just a bit bigger than A6 (4x6 inches, 10x15 cm). And I NEVER had a problem with such small envelopes, the only problem could be if you know that the celebrity would send you large photo, so you have to enclose big SASE. But if it's known that the celeb sends small photos or you send your own photos to sign, you can definitely use smaller envelopes {thumb2} .
On the second envelop i put my address two times (sender and receiver) and i write 'do not bend'
It's not necessary to put your address on both places, but you can do that. I never did that, though, so I don't know if it makes some difference.
In the second envelop i put the IRC's (or do i have to put them in the biggest envelop?)
You can put them wherever you want (into small or big envelope) , it's up to you, just make sure the celeb will find them

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For an envelop (size 9 x 12) to Belgium, how many IRC's do i have to include? Would 5 be enough?
I never sent such a big envelope, but I think 5 would definitely be enough

. But it depends on the size and weight of the envelope and the country from which you're going to receive it.
And concerning the LOR, does it have to look like a formal letter?
If you are a fan and don't collect for charity, it doesn't have to look like a formal letter, but be sure to be polite, thankful and don't use too much familiar language

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I hope I helped a bit
Sammya