How to fill a smaller video_frame image into a larger window?

For example,
Firstly, I have known the below:
unsigned char * pBuff; ( the pointer to data buffer, it is successfully initiated at before)
int nBuffLen=230400; (3202403)
int nFrameWidth=320;
int nFrameHeight=240;
int nPixedBpp=24; (Frame data’s Format type : RGB24 i.e.3 bytes)

Secondly, I want to fill the video image into the whole window which is 640(width)*480(heigh), how to write the code? Should I build a SDL_Surface which is 320*240*24 or 640*480*24?
Thanks a lot.

Best Regards
Zhou, Yubao
E-mail: @Zhou_Yubao