Trung, the beds are 4ft x 8ft x 1ft tall. If you buy three 2"x12"x8ft lumbers you can cut one in half to make the 4ft end pieces. Its just a rectangular box a foot tall filled with compost, dirt, food scraps, fertilizer, what you can throw at it.
You dont really need raised beds even, watermelons like just a big mounds of compost and rich soil. They are heavy feeders. I used a lot of compost and some avocado fertilizer to start then a potasium nitrate with minors fertilizer once and a potassium heavy slow release for the fruiting stage. You can throw a lot of fertilizer at them if you have good sun and good dirt they will take it. And lots of water.
It takes a month to get seeds going in April and then transplant them in May or June and melons are ready in August. The fruiting times are listed on all the types in the link I posted. It looks like my vines are setting more fruit now so assuming they dont get any disease they will probably produce for 2 or 3 more months.