That's really a factor of how many sts you cast on, and the cast on is always going to be about 80% of the width of the knit item, no matter which CO you use. Using a bigger needle for LT usually just makes the loops on the needle larger, but doesn't affect the loops from the thumb. The stretchienss of this CO comes from the tension on the thumb yarn and the spacing of the sts; so don't pull it tight and leave at least a yarn width between the sts instead of snugging them up close.
You can use the single loop version
of Magic loop and pull out a loop of cord between stitches to get the ends together. Try out other cast ons from the Cast Ons page, just don't pull the yarn tight when you make them.