Are you sure you want to put down gravel? My last spring's raised beds -- 2 cedar 2x4's stacked -- unfinished compost, cardboard and straw flakes directly on lush lawn plus mixture of bagged topsoil, mushroom compost and bagged compost called Bumper Crop -- topped with additional home made compost thru the season and fall.... THIS spring, I could stab my gardening fork all the way in -- and std. gardening fork tines are about 10~11" long -- without stepping on it. My subsoil under 1" of sod and 1" of topsoil is -- or should I say WAS -- pure clay. If you put gravel down, you won't be able to use the naturally improving soil underneath.
I admit my raised beds are on a slope (about 5Ã‚Âº) so I didn't have to worry about drainage -- I figured moisture will just wash right down.
This year, I've added another step -- fracture the soil with gardening fork BEFORE piling everything on top.
I used 12' and 8' lowest grade cedar 2x4's from Home Depot. They'll cut them up for you in any length you need as long as all you need are straight cuts. I went with it because I'm not handy and I wasn't about to try any fancy angled cuts to make everything plumb and level -- and it worked out because now my boxes lay at the angle of the slope facing SW.
Someone in last year's thread said he uses regular (fir? pine?) 2x4's and just replace them every 4 or 5 yrs or so.
FWIW and FYI -- my beds are in 14'x10' area against a fence corner -- 2'x10' + 2' path + 4'x10' + 2' path + 4'x10' (plus another 2' walk area on the other side so technically 16'x10' I guess) I wish I'd laid things out a bit differently now though, so I don't have to walk around the middle bed. I've compensated by laying a 4' 2x6 board in the middle bed so I can short-cut across sometimes. I'm thinking of using that for my summer sprouting area later on (a trick I read somewhere to sprout fall crop seeds under a board in the heat of the summer)