Is Mima yarn a bulky weight yarn (a class 5 weight on the label)? I couldn't find the yarn to check. The yarn used in the pattern is Lionbrand Woolease Chunky, a bulky yarn.
If casting on 138sts made sense from the measurement of your gauge swatch, you should be ok. It's a little difficult to judge the length while the scarf is on the needle. You could take the scarf off the needle by threading the sts onto a good length of scrap yarn and then measure the length or try it on to see if it's right.