What kind of grow beds.
Well you later say you need to build rafts so I'll guess that means raft grow beds. So, 4x16 or 4x20 Well if you are making the beds actually just a couple inches wider than 4' then stick with multiples of 4x8 leaving yourself just enough more than each length to accommodate actually putting 4' wide rafts in and out without needing to trim them down to fit with the liner and all. So I would probably recommend like 50 inches by 196 inches if you want to have rafts in the 4' by 8' range (though most people find cutting them down to 2' by 4' makes them easier to handle.) Going up to 20 feet long will actually make the raft boards a bit odd for the beds but depending on how many beds you have total if you cut down to 2x4 for the rafts this might not be an issue. The dow blue board that most people use for rafts come in 4' by 8' sheets.
The 2" thick stuff painted is what the commercial scale operations use. Other things may be easier for the smaller scale home (non commercial) production.
For gravel grow beds. I'm short so 4 foot wide beds are a bit much for me to reach across.
And overall size for gravel grow beds should be more linked to the flow through characteristics of the media and the flood and drain technique. I would probably recommend 4' by 8' grow beds if you are married to the idea of 4' wide. 4' by 8' gives you a nice size to work with most lumber building materials. However I found that you might also need to think of the liner material also. A 5.5' wide roll of liner is going to be a bit narrow to manage a 4' wide bed and still be deep enough. I did my beds about 32 inches wide for that reason. However definitely a metal stand is a good idea when thinking lumber and liner since termites were a problem for me and now I use stock tanks.
David makes a good point.
Sequential pumping with timers needs to be carefully done since separate timers can rarely be expected to keep perfect time.
An indexing valve can be used to pump to beds in sequence but a bed that big will take quite a long time to drain.
Thanks for the info. Now I guess I'll make 2x beds andtake another look at my lumber pile. I tend to buy large amounts of items I may need and then when I'm broke I can keep on working. While working in the yard the other day I realized I could have some grow beds on existing fish ponds . made a test run today and wont need to make a sump like I previoulsy thought. Just move some fish and put in pump to GB. Thanks again.