Complex silicon polymers generally fall into two classes: those that are inert in water, or those that are unstable in water.
The same applies to simpler silicon compounds: silica is almost entirely inert in water (it does dissolve as silicic acid, Si(OH)4, which makes it bioavailable... but only in very low concentrations, and no naturally-evolved terrestrial organism actually breaks the Si-O bonds to use it for anything more interesting than redepositing silica structurally); and silane, SiH4, rapidly hydrolyses.
So, simply by knowing that your aliens are silicon-based, we can immediately conclude that they do not use water, and they do not live in a similar temperature/pressure regime as we do on Earth. You won't be shaking hands with them without one or both of you being in a pretty hefty spacesuit!
Your aliens might well be extremely cryogenic, existing in liquid nitrogen and/or eutectic mixtures of organic solvents. Or slightly less cryogenic, relying on high-pressure anhydrous ammonia. Or somewhere between "uncomfortably warm" and "lethally hot", with bodily fluids based on fuming sulfuric acid.
Unfortunately, there's not much farther we can go from there. All of these fluids are transparent, so there's no barrier to fluid-filled eyes like we have--but also no barrier to pit-eyes or compound eyes. None of them are associated with atmospheres so dense or gravitational environments so different from ours as to significantly alter the mechanics of walking. None of them necessitate that large creatures be cold-blooded or warm-blooded. None of them would constrain the colors available through structural or chemical pigmentation. And similarly, this level of detail provides no chemical constraints on what the ecology in which your aliens evolved might look like, so you can't make inferences from that, either.
So, basically... they can look like whatever you want, as long as you can justify that appearance according to other constraints, common to all sorts of life.