-The word count is not the most important thing. It is your blog, your review, your opinion. It is not a college assignment that it should have a word limit.
-The best idea to write a review for an app or a game is to "develop it as it goes" like you mentioned.
-The idea is to give your views about the app or the game.
-A couple of things that readers might want you to include in your review are:
- The main tasks that the app can do (what is the app supposed to do)
- How good is the app at doing what it is supposed to do
- How "smooth" is the app (how smoothly/effortlessly would the app run under specified conditions)
- What are the minimum requirements for the app
- Is the app paid or free
- How does the app compare to similar apps (names of similar apps, is this app better than them etc.)
- If the app is paid, is it worth paying for?
- For how long would you expect the app to stay in the market (is it something novel that may not have competitors for sometime or is it just another app that has come and will go)
- Any problems or missing features with the app
- Anything unique about the app that others do not have
- Any updates planned (if you have info on this, that is)
- A rating out of 5 or 10 (for some reason people like rating out of 5)
- Your final verdict (to go for the app or not)
-Write it in free flowing manner that comes naturally to you. It is your blog, after all. You are not writing an academic paper so you do not have to plan like one. Blogging can be done free style.
Hope this helps! If you need more details, feel free to say so!