Generally when you buy a sail you will get the information on what boom/mast is required on the sail or the sail bag.
when you buy a board the shop will tell you the recommended sail / fin sizes or you can look them up on the manafacturers web site.
Best to go to a windsurf shop for advice, if you've got one near you!They will make sure you get exactly the correct gear for what you need to get going, and if you don't want to buy new, they always have 2nd hand gear.
Also go down to you local beach and ask some experienced windsurfers for some info.