IELTS,band 5

I wonder, in order to get band 5 at Ielts exam(GENERAL,not Academic), how big vocabulary one must have?
Any suggestions?

Basic vocabulary. These sites might be of some help:

http://www.ielts.org/pdf/UOBDs_SpeakingFinal.pdf

http://ielts.org/PDF/Half_band_updat..._Materials.pdf.

See also IELTS Exam Preparation - IELTS General Tests and Vocabulary (scroll down to where it says vocabulary strategies).

Also remember that vocabulary is a tool not an end in itself- simply having a huge vocabulary does not guarantee a result.