I'm quite a big guy and I'm travelling to Benidorm and worried I won't fit in the seat when the armrest goes down. Shall I just buy another seat to save hassle when on the plane ?
Can you measure your width? Some people are wider at the hip and some people are wider at the shoulder. The typical width of short haul economy seats are about 17-18". If you're on an airline with so-called business class, in Europe this is typically the same seat but with the middle seat left free. In that case you could raise the armrest and have some more room.
Seat width for various aircraft of various airlines can be sean on seatguru amongst other places.
It certainly is possible to buy an extra seat on most airlines, and in some cases it costs less than buying two full seats since you won't normally be paying some/most of the taxes at least.