If you move your hands fast and slow your machine down, then your stitches will get bigger. If you speed up your machine and slow your hands down, your stitches will bet smaller.
Shredding thread could be due to a number of different issues. The first thing I would do is switch to a 100/16 top stitch needle to see if that helps. Also try re-threading your machine. Shredded thread can also happen if your machine is going too fast for your hands-- taking too many small stitches in one place can sometimes pierce the thread, causing it to shred. And sometimes sudden changes of direction or speed of your hands can lead to shredding thread if you get out of synch with your machine so the fabric is being moved after the needle comes up, but before the stitch is actually completed when the thread comes off the hook.
Depending on the kind of bobbin you have (horizontal drop in, vertical front loading, or vertical side loading) your machine may be a little more fussy when FMQ in one direction and that can also sometimes lead to thread shredding. Try to figure out if the thread is only shredding when you move the fabric in a specific direction, if so, then just slightly angling the fabric when you need to move it in that direction may help.
Rob