Do not be alarmed if the size does not fit into the slot. Operators have adapters of different sizes for each of the options.
Before changing the SIM card, make sure that the iPhone is not blocked by a Russian operator. Such a nuisance happens when buying a smartphone that has been in use or has been in the hands of a miracle craftsman. Do not try to crop the card yourself to the desired size. Such experiments will only lead to damage to the SIM card, and at the same time the slot. If in doubt about your own abilities, contact the sales center or service, where service workers will perform all the necessary work.