Updated FAQ (markdown)

FAQ.md

@@ -345,6 +345,33 @@ Reusable         | no<sup>(3)</sup>   | no<sup>(3)</sup>
 
 As a general rule, if your `StaticJsonBuffer` is bigger than 2K, then it may be a good time to switch to a `DynamicJsonBuffer`.
 
+### How to determine the buffer size?
+
+There are basically tree approaches here:
+
+1. either you can predict the content of the object tree,
+2. you know how much memory is available.
+3. you try and look at current size
+
+In the first case, you know some constraints on the object tree. For instance, let's say that you know in advance (and by that I mean "at compilation time") that you want to generate an object with 3 values, one of them being an array with 2 values, like the following:
+
+    {"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}
+
+To determine the memory usage of this object tree, you use the two macros `JSON_ARRAY_SIZE(n)` and `JSON_OBJECT_SIZE(n)`, both take the number of elements as an argument.
+For the example above, it would be:
+
+    const int BUFFER_SIZE = JSON_OBJECT_SIZE(3) + JSON_ARRAY_SIZE(2);
+    StaticJsonBuffer<BUFFER_SIZE> jsonBuffer;
+
+In the second case, let's say you dynamically generate a JSON object tree of a random complexity so you can't put a limit based on that. But on the other hand, you don't want your program to crash because the object tree doesn't fit in memory.
+The solution here is to determine how much memory is available, or in other words how much memory you can afford for the JSON object tree.
+
+The third solution is to run your program an print `jsonBuffer.size()` to get the current size of the buffer.
+
+**WARNING**: if you use `String` to create your JSON keys or values, there content will automatically be duplicated in the `JsonBuffer`, so you need to add the total length of all strings in the size of the `JsonBuffer`.
+
+See issue [#243](https://github.com/bblanchon/ArduinoJson/issues/243#issuecomment-196553398)
+
 ### I found a memory leak in the library!
 
 This is very unlikely. You're probably using the library incorrectly.