Videos not showing the right size

[motomibo]

Hello,

We are showing brief videos as stimuli. They were exported at 640x360 size. We would like to show them bigger. In the input file I've entered the videos a few different ways. For example:

"movie1.wmv" (640x360) : shows normally at that size
"movie1.wmv" : shows normally at that size
"movie1" : does not show

Which is the right format? Why?

Then I tried entering larger sizes (same aspect ratio) in the brackets following the name of the movie, such as "movie1.wmv (1280x720)

Some of the different sizes work, some of them don't. But the main problem is when the video starts, there's a flash of the first frame in its original size, and then it "blows up" to the specified size. It's a noticeable problem and we'd like to get rid of it if we could. Any suggestions?