There are different sizes for SIM cards. The whole old SIM cards that had a quite large size, were replaced over time by Micro SIM cards, which are somewhat smaller and recently, there are also Nano SIM cards. Special Apple opts for Nano SIM cards.
There are also adapters for the various cards, but we always recommend the size that is needed in the mobile phone. Usually, you can order a Micro or Nano SIM card from your provider free of charge.