What do you weight in KG?
Board sizes are always measured in volume of liters, basic rules is that a board needs to be equal to your weight in order to float when not planing.
Length is not that relevant, actually short and wide boards are a bit more stable.
You would probably need a board of around 150 liters, that will be big enough to stand on and lift the sail out of the water (test some to see what you like).
For a first sail I would get a 5.5m sail, anything bigger will be hard to sail without a trapeze.
Rule for sail sizes is that your next sail must be around 20% bigger (5.5-6.6-8 for example) thats why there are so many different sail sizes.

Someone posted the Starboard website earlier, their Carve line of boards are perfect for beginner and intermediate windsurfers.