You know...its so hard to say whether or not to ride because I've ridden M:S over 20 times now, doing all the stuff they tell you not to do, and I've never had a problem.
It is definitely a fairly enclosed area, but you do have your personal space. There is a feeling of depth in front of you b/c of the video monitor in front of you. The front of the capsule does move in to close, however, there is plenty of legroom and there is room to breathe.
I would say, check out the sample capsule in the M:S courtyard as it will give you an idea of how much room you'll have.
One final thing...please DON'T close your eyes during the training exercise. Doing so will only exacerbate any motion sickness that you may be experiening. Find a point directly in front of you and focus on it to minimize any sense of disorientation due to the spinning.