There were two important design criteria.
1. Length and width must be a couple mm's smaller than the length and width of the mft measured to the side rail faces. This is so you can clamp to the side rail faces as the picture shows.
2. The mft top should be a few mm's below the surface of my table saw. I also made it enough low so that if I fit it with stub legs to raise the wheels off the floor, it will still be slightly below the surface of the table saw. I think this ended up making the top just slightly higher than it was with the legs on it.
I can't see the end grain, but typically beams like that are cut from the center of the tree. That will diminish the value because you can get fewer usable boards from it. The pith of a tree is not stable.
I typically pay <$2bd ft for air dried, sub FAS grade domestics, regardless of size.
I got a phone call on Wednesday from my barber, he said he had a client who wanted me to make a cigar box guitar for him, he has a box he wanted made out of, I said go on, would I be able to make one for him? I said no problem, then he said the downside is it need to be done in two weeks, it is going to be a gift for his father, I said no problem, picked up that night, I have the neck glued up shaped and ready to be installed, today, Most times I take my time making CBG's most time I will take up to six weeks, I was lucky that I had enough supplies for this one, except a sound hole cover, which is on order from Gitty,