what you'll be best of doing is putting the video clip onto your computer and then compressing it instead of resizing it. i use soreson squeeze and if i have to send a little video ive made i will compress it.
or do you want to change the resolution? if you do this it'll make the video smaller and less likely to be watchable as it will probs be of 320x240 for example. so i reccomend compression. and also how long is this little clip?