I'd say full size, or maybe 7/8. A full size will be easier to find. Also consider your hand size: if you have unusually small hands, you might be more inclined to go towards 7/8. I also don't know how old you are, but if you're expected to grow, full size might not be a bad idea. I'd most likely say full, because there are advantages to playing a more "normal size" cello, like finding the right strings and cases. Larger instruments tend to have a bigger sound, so that would be another plus.
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