Emanuele,
I posted a spreadsheet to help people estimate the memory requirements for a TurbSim simulation. The link is in this post: Memory Requirements for full-field wind arrays - #3 by Trevor.Demme, but earlier in that topic is a summary of what that spreadsheet calculates.
If you are using a 32-bit TurbSim executable on Windows, you will be limited to a total of 2 GB (of RAM) in your simulation. If you need to simulate something larger than that, use a 64-bit executable.