My bed is about 6'x3' due to the pickets I had on hand. The slide portion would not be necessary. As the potatoes grew I just added dirt until it filled up. Since I got the bed in late it did not grow as high as it would have if I started earlier. I would have just added more boards and dirt as it grew.
I heard a podcast yesterday where they priced a raised garden kit, dirt, mulch for around $300 installed. Their calculated payoff was year 3 for that. You could build this 5'x5' raised bed for about $50 with the dirt and seeds. It's an easy way to get going and should payoff the first year.
I really prefer growing in the ground directly without raised beds. I am going to experiment more with the raised beds for my potato growing. Next year I only plan to have a 5'x5' spot to maximize how much I can grow in my limited space.
If you would like for me to set up a raised bed for you and get going please let me know. I can install it on your site for a fee and I would love to do this for you. See my contact info on the site.