Just to confirm whats already been said,a 1.8m dish will get you most of the channels but you will still lose some at certain times of the day and night and at different times of the year.Smaller dishes still work but I wouldnt go smaller than a 1.3m.
Lots of sat men out there but one you can trust is called "Jurgan" (The German) who is fairly well known on the Island and I should think he still advertises in the local ex pat rags such as the "Ibiza Sun"
John.
P.S. Dont be tempted to go bigger than a 1.8 as the 2.4m doesnt work as well.......