We don't have any limits to how much code you are allowed to post. The more code you provide, the more description of the code should accompany it.
Be aware though that there are two limits that you need to be aware of:
- The code you want to have reviewed must be contained within the question itself
- There is a limit of 65536 characters in a Code Review post. (Most Stack Exchange sites allow just 30000.)
What this effectively means is that you can have approximately 30kb64kb of code in your question. If you want a review of more than this, divide it into several parts and several questions.