There's probably not a great deal I can add to this, but as I play both standing and seated, I'll offer my thoughts.
Initially, I did my first nine months of learning entirely seated. Oddly enough, the main reason was the same health problem as Gordon, (which is also why I chose a theremin when I wanted to learn a new instrument - because I knew I wouldn't be able to manage something like a violin or flute that I'd have to hold; the non-contact of the theremin suits me/ME ideally).
Anyway, I found that playing while seated helped me with stability. Wedging my back firmly against the back of the chair, and anchoring my right elbow against my right hip seemed to give me a pretty stable right arm (I followed a playing arc dictated by the pivot of my right elbow, if that makes any sense).
The down side of this was that I discovered that knees can interfere with the fields. I soon realised that with my left knee where it would comfortably be while sitting, it almost muted the volume loop! I would therefore sit with my left foot tucked under the chair to lower my left knee ... however, it took me longer to realise that my right knee appeared to have some affect on the pitch field too. It was nothing major, (and nothing I wouldn't be able to adapt to now that I'm a little more experienced) but I eventually noticed a difference in my note-reliability depending on whether my right knee was "up" or tucked under like my left knee. It seemed that the insertion of my right knee into the field subtley altered the field shape, so that despite careful tuning and playing the same arc, results changed a little depending on knee placement. As I say, it wouldn't throw me now ... but I know it did then; personally, I felt I needed everything consistent while learning, although I'm more adaptable now I think. I'll add that seated with both feet tucked under the chair could be less comfortable than standing.
I suspect I may well have stuck with playing seated, but for the arrival of the E-pro and, shortly after, the Wavefront theremins. Initially, I found I swayed all over the place and really hated playing while standing. I bought an adjustable stool (not unlike Randy George) and used this a lot at first. However, with practice I eventually found that I prefer to play standing. (Actually, there's no choice with the Wavefront. Unlike the Epro which you can sit at with one leg either side of the stand - the Wavefront cannot really be played seated).
Perhaps, in this respect, you could do whatever feels most comfortable and appears to get the best results at this stage in your learning. Whereas most unsuitable habits picked up while learning the theremin can become ingrained and not simply remedied, I suspect this is something you can easily adapt at a later stage without it setting you back. However, if I had to pick and recommend one route, I'd say stick to standing if health permits!
