Jon,
Depending on the size of the person, I suppose. For me, it would be about 24 inches. That would allow me to freely work and lean over into the bed. Also, if stacked securely, one could sit on the edge.

This is very informative: http://www.hort.vt.edu/human/pub426020d.html

I am hoping to have at least one or two this year, due to my health.

Take care,
Barbie

Jon,
Being in a wheelchair I have my raised beds at about 10"
as I used 2x10 planks. It seams to be a good height for myself. The depth of them should be twice the distance you can reach as what you cannot reach from one side go around to the other to reach the side you missed.
I wouldn't go over 20" as remember you have to reach up to pick. If you make them 32" and the pepper grows 12"(a small pepper plant) you than must reach up 44" to pick the top of the plant.
Measure the reach distance your have sitting down and make the beds to fit you.

Jerome

Greetings All! Jerome, thanks for your input being I take it you are already in a chair. Barbie, thanks for the website address. I have already bookmarked it.
Jerome I guess I was still thinking from the standpoint of still being mostly upwardly mobile. Do you do most of your gardening from a chair, or do you get out of the chair and sit on the ground or other structure? I also wonder if the figures that you gave me would apply to greenhouse benches and or greenhouse beds?
Thanks for the feedback you have given thus far. Have a great week and a great garden! Jon.

Jon the dimensions are what I use outdoors.They could be used in greenhouse but will you be doing only greenhouse gardening?

If the greenhouse is only for starting the plants than I would make tables that are a height you are comfortable with while sitting.These tables could be mini-deep,about six inchs, to allow you to start the plants and than transplant them to your outside raised beds.

I had a spinal injury, use a manual wheelchair so getting on the ground and than back into the chair takes a helloflot of engery. Never mind the friction sores I would get by sliding/transfering to a different spot along the pathway than the oh yea I need more seed which means getting back into the chair getting the seed than back into the garden and on the ground.Man I'm getting pooped just thinking how I would do it.

Height of greenhouse sitting benchs are the height that is easy for you to get onto. Working bench will be the height of 27" with enough room under it for you to pull yourself close up to the bench.The same would be for the above table.

I have my raised beds at 24". This perfect for me as I use a power wheelchair. Just be sure to leave a 4' space between the beds to allow access & keep the beds narrow enough so you can reach the center from both sides without leaning over (just try bending over working on a raised bed for about 3 hours to find a new pain limit).
FYI: The Mantis roto-tiller is great to use as it weighs just 20 lbs & is small enough to handle from a wheelchair.

Plant Manager:

I politely suggest that 20' long is too long. If you have the room, halve that and put in those 4' alleyways as already suggested. It's not like being able to lay a plank across the span and walk across to continue a planting block. There's all the trip back, round and return to where you were, but opposite. Whew. No fun on a showery day.

I've seen pictures of very raised beds that have room to tuck knees and chair under. The extra plus on those is being able to use them like a raised cold frame and growing more tender plants. You might want to consider such a treat now while you can customise it to your satisfaction and stretch.

I have cinderblock (8x8x16) beds. I have two big "L" shapes that face each other (one backward) and then two squares. I didn't have a mason do it, just landscapers, so that if I want, we can change it later. We laid black landscaper mat on half the backyard, then laid landscaping cloth (that's metal mesh) in the areas we were building (to help keep out gophers tunneling up from below), laid a row or two of cinderblock, dumped in some gravel, finished the brick, and then a bunch of topsoil, and then for the top 16" or so, a ton of mushroom plant compost.

Half the L's are 4 bricks high (32") and half are 3 bricks high (24"). You can use the holes in the bricks either for little plants like flowers (but warning, the soil dries out superquick), or put flat toppers on the sides which looks really nice and allows you to just sit on the side of the bed (the 24" ones). I find the 24" ones hard when I am standing as it is leaning over, not enough to be kneeling but too much to be standing. The 32" ones are better for when I am standing, but then, I can't sit on the edge, either. All the shapes are around 4' wide. The two smaller squares are strawberry beds (four diff kinds). The two big L's are veggies and herbs.

I have five cattle panels (4" metal square grids, 4' x 16' usually) that are bent over in arches along the fence as strong trellises. (We get tornadic-strength straight-line winds here, so a strong trellis is a must.)

I made my own so-called earthboxes(tm), for pretty cheap really. I put trellising plants like cukes and squash and so on in them, water them well and they work just fine.

Realistically, you can do container gardening to much more extremes than people realize. I have grown a bell pepper plant in a 4"x4" (x16" deep) cinder-block square! OK, it was quite small (the depth helped), and the peppers were small, but the point is that you CAN, if you must, grow plants in a much smaller amount of soil than most people realize!

Palyne