Windows 7: Youtube

I was wondering what is the best video size to upload to youtube like 740xXXX

Youtube recommends 640x480 resolution.

There's a lot to be said for uploading the best quality your camcorder of whatever you use to produce the video file can manage. You Tube will render it down anyway so start with the best. If you are putting HD clips on though best to keep to You Tubes recommended size. the file format matters a lot too. here's You Tubes recommendations:
There's full details and some useful guidance on a page on You Tube. You need ti sign in or register if you do not have an account. Click on Upload then scroll down to "Best Video Formats for You Tube".
Post back if you need further help.