Ahhh, that's the question, isn't it. :) Unlike the Apple devices there are a prolifera of Android devices having different shapes, etc. This makes it difficult for the retailer and I understand that. So let me tell you what I have now and that may serve as a guide.
I use a Samsung Galaxy Nexus phone as my Android device. Anything larger is just too unwieldly for a knee board. In any case the display is about 300mm from my eye so nothing larger is needed.
The knee board I use now is one I made myself by taking a "universal" holder and bolting it to a plain knee board. This works fine but is a big bulkier than I like, mainly because of the universal holder. I bought the parts from an overseas supplier - I won't give the links here. If a better purpose built product was available I would be interested. And, of course, I always prefer to do business with an Australian retailer when ever possible.
I hope that gives you some ideas.