how to make the wav file MB to be KB because the file is too big !!!

buy a memory card or setle for poorer sound quality. I use quicktime which gives me loads of options for exporting wav files. Im sure there will be plenty of free apps that do it. Try having a read around the forum...

You can use a WAV editor like Audacity to change it.

You can't make it significantly smaller unless you make it significantly shorter, or unless you convert it to some other compressed format (AMR, AAC, MP3, Ogg Vorbis, etc.). The compression is lossy (i.e., data/quality is lost).