when i resize the barcode according to james shortcut it seems too small .thats how it supposed to be ? or it must be the same size as the one in the MS Word copy of the I-864 coversheet ?

What causes this is when you crop'd it out of the image, you picked up some white space around the barcode.
Then when it pastes into the word document, it looks smaller.

You have 2 choices:
1) crop closer to the actual barcode when you copy it.

2) make it bigger in word.

To be honest, so long as it looks reasonably close, and scans with their reader, you will probably not have any problems.
There is of course no guarantee, but no matter what you do it wont be the exact same as what they mail.

Ive been trying to find james short cuts but i cant seem to find anything except people mentioning it, is there a link or something, I've looked in wiki section but still cant find it. thnx for your help.

I've done a little work with barcodes. Be careful when resizing or cropping a barcode. Resizing it may be OK, as it's usually the ratio of the sizes of the white and black areas that is critical. However, if you resize it smaller, the size of the elements can become too small for the reader to distinguish. Cropping can also be a problem. The white space around the barcode is called the "quiet zone" and making this too small will cause the barcode not to read at all. Any black in the quiet zone is a big no-no as it must be all white.

Here is the link for James shortcut

http://www.visajourney.com/wiki/index.php/Jame%27s_Shortcut