Mower width is important if you plan to mow between the beds, and that's usually 22 inches for a walk behind. Most people cluster beds with narrow pathways between them, with wider corridors down one side for easy access with carts, etc.

No sales pitch. I'm not affiliated in any way. Just a cool tool that can help a newbie lay out his container and info on numbers of plantings and care. On the link in the above thread go to the upper left, three little tomatoes that says,'New!Kitchen Garden Planner' Then when the box pops up that says, 'Plan Your Own Garden' press 'Get Started' and you're good to go. Try the link below Here is a link that might be useful: Kitchen Garden Planner

Excavating any kind of bowl in the clay and backfilling with anything, and particularly anything organic that will decompose over time, is a mistake. You'll eventually be left with a "bowl filled with dissimilar materials" from the clay and it will fill with water and be the soggiest part of the surrounding area each time it rains. You want to eliminate the bath tub effect in clay - not create it. Create your raised beds on high spots or create high spots by adding native clay before you build the raised beds. Others with more experience can guide you on the rest of your questions. Al

There are a few "easy" options that should help fix this problem. 1)Let them sprout, then fertilize, wait a couple days, and spray with round-up. Continue to water until they grow themselves to death. Then no weed seeds and the weeds are dead 2)Build your raised bed. Cover thoroughly with cardboard (will break down after the weeds are dead and not be annoying when cultivating in the future) and then soil. Careful not to puncture cardboard in your first year. 3) A firebomb type attack in which you use #1, then put down preen or casoron, then follow through with #2. This is likely to be overkill, but also likely to be the most effective method. You may have poor success with things growing for a few years in this bed as a result, but you're also unlikely to have 4-o'clocks growing in this bed either, which is the intended goal.