For teeth to mesh they must have pretty much the same spacing (teeth per inch) on each gear (sun,planet,ring) and so the radii of the gears are in proportion to their number of teeth.
Since the teeth interleave the question arises to what you take as the radius: the internal radius (innermost cut), external (furthest metal), or 'somewhere between' makes more sense, but precisely where?
A planet gear will be half way between the edges of the sun and ring gears.
The rest will be spread out most sensibly opposite each other (if there are 2), in an equilateral triangle (if 3),in a square (if 4) etc.
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Are you expecting a detailed answer dependent on tooth profiles, tooth gaps,etc?
Besides what crysta1c1ear points out, your need to select the appropriate numbers of teeth for each gear in the planetary so that it will assemble.
Beyond those basic issues, the pitch diameters, tooth profiles, etc. of the gears are also routinely modified from standard for optimizing parameters like recess action.
Finally, it's not really the geometries the gear set has when idle that matter. It's the tooth contact geometries that result when the gears are fully loaded that count. For this reason, high-performance planetary designs commonly use techniques like flexure-mounted ring gears and spherical roller bearings for the planet gears, in order to equalize mesh loads.
