Fujifilm instax film became very popular in the market and its one of the best options. There are 2 format film: Instax Mini and Instax wide. And if you are not sure which format is better for you i will try to help you choose:

Instax mini - as the name suggests is a small 5.4(W) x 8.6(H) cm | 2.1' x 3.4' credit card size photo. Its a portrait type film that has thicker white border in the bottom of the photo. So when you slide the photo in the pocket your photo wont be covered. Also its the cheapest film. Price for 10 pack starts from 9$. Fijufilm cameras that uses this film has a portrait type lens so the best to shoot portraits of 2 people. Yes the size of the photo is quite small but it has something special about this format. It feels like a selfie instant photo. And if Polaroid created their legendary square format Fujifilm definately has their own now!

Instax wide - as the name suggests its bigger format then instax mini. Photo size is 10.8(W) x 8.6(H) cm | 4.2' x 3.4'. So two Instax mini = Instax wide photo. This film is horizontal or landscape format and its great for bigger group photos. Cameras that uses this film has a wider lens so this a great choice if you want to make several location shots too. Film is more expensive then instax mini, 10 pack starts from 13$ - but HEY you get 2times more photo space! 

My suggestion to pick the one you like better. My favorite is Instax mini but i doesnt mean that Instax Wide is not great choice! Dont worry, both of them will work great for your celebration!