Go fot the twin size and be sure to add enough on the sides to hang down & cover the mattress when the bed is made.
A 38 x 70 bed would need to be about 58 inches wide, if you use a dust ruffle. Add another 20 inches if you don't so it will hide the box springs too.
As far as the length. . . Are you going to make it long enough to cover the pillow? or make matching pillow case? The length would be about 85 inches with dust ruffle, 95 without, plus what you would fold down or cover the pillow with, another 20 inches.
I would make it 78 x 105.
I have made several, for kids & now grand kids. The ones my grand kids use the most are those made from that microfiber fleese, It is basically blanket material, but much softer. It comes 60 inches wide. I use to get about 1 yard, when they were little.
They can carry it from room to room where ever they are and sleep on it or under it. My grandson is now 5 & I bought 2 1/2 yards to cover his bed. In the summer he sleeps on it, in winter under it. So much for the quilt I made:)