14' x 13.6' should not feel tiny unless you're accustomed to having a much larger bedroom.
Try mapping out the room on graph paper (or computer software). Note the locations of doors and windows, and be sure to include door swings. Then draw in (or make little scale cutouts of) your beds and other furniture. That will give you some idea of clearances and whether you have too much stuff for the space.
We just put an offer on a house with a master bedroom that's 13x13.5'. We have a queen and a twin bed, and I figured it's small, but sufficient. A lot of the houses I saw had even smaller masters. I'd think with 2 twin beds you'd be fine.