I've been told they're is different SIM card sizes, and I just wanted to know would it fit?
Does the SIM card that's inside an iPhone 4 fit inside a Samsung Galaxy S3?
Yes! Both Iphone and Samsung S3 have use a micro sim, which is a much smaller sim than a normal sim card.
I know because i have the S3 and my friend has the Iphone 4 and we have looked. Hope i helped! :)
Like the first answer said, yes they're both micro SIM cards. I was a bit shocked when I saw that the s3 used a micro sim too. All the other galaxy's were standard sized sims.