hajdii:
Firstly thank you for giving this subject any thought at all and trying to inform yourself. Most people wouldn't bother.
Like offstand said, easyjet (and I believe most airlines do) carries extension seat belts so there is no need to worry about the length of the fitted seat belts. Just take a seat, try to use the seatbelt and in case it does not fit ask one of the cabin crew for an extension. They should provide you one without making a fuss about it. They are easy to fit but if you should have any problems the cabin crew can easily help you.
Make sure you take a seat with an orange headrest as the seat belts are slightly longer in those and in case that you do need an extension the cabin crew won't be able to allow you to sit in a gray head rest area which might mean quite a bit of hassle for you (and the crew) trying to find you another seat. As well, if you feel you might struggle with one seat it might be as well worth considering to purchase an extra seat as flights do get fully booked sometimes, especially in summer. However, if it's only the seat belt length that causes you concern don't worry, that's no problem :-)
I hope you'll enjoy your flight :-)