I hate to get out a big piece of batting to use for a quilt that is smaller than the batting in the bag. So I try to take some larger "small" pieces and put them together. As long as the batting is the same type you can do this. You should not use dissimilar batting types because they will not be even inside the quilt.
Take two pieces of your batting and lay them side by side. If the sides are straight you can join them by butting them up against each other and using your mending stitch (a zigzag stitch on your machine) to join the pieces. Do not overlap them, merely butt them up to each other.
If the pieces are not straight, over lap the two pieces slightly, maybe 2 or 4 inches. Using a rotary cutter, cut a gentle S curve all the way down the length of your pieces.
Discard the smaller pieces and butt the remaining larger pieces together. The curve is like a puzzle and will help you keep the pieces straight as you use the mending stitch,
If you are careful not to stretch the batting, you will have a larger piece from your smaller pieces.
Oh, use a beige or white thread and you won't see a thing! I also use a lint roller from the dollar store to get all loose threads from those pieces of batting that have been stuffed in the Batting Pieces box!