i'm starting out

hi all,
ive decided to start out on the garden, ive got myself a sit on lawn mower to cover the half acre of garden. i have put a brick river down the centre towards the drive. i have built set of pillars and hung a set of gates. i have a garage half way down the land, at the side of the garage i'm thinking of building a raised garden. what size would you think is a reasonable size. i would much thank any body who could reply with any photos and comments. thank you
benny