it's not clear if you are trying to play one video or choose from multiple videos. there are so many different approaches to building video players, from fairly simple to quite complex. many professional multi video players are xml driven.
that should give you a good idea how to build one using flash's built in components.
you ask about the opacity of the seek bar. if you are using a skin on the video component, the opacity of the skin can easily be adjusted through the component inspector. as you are choosing the skin, you can also pick a color, and adjust its opacity/alpha.
if instead of a flash video component skin, you use the SeekBar component, then you have to give it an instance name, and then in the actionscript panel you change its opacity via actionscript: seekBarInstanceName.alpha=.5; for 50% opacity.
if you've built and coded your own seek bar, then it is a movieclip like any other movie clip, and you change its opacity using the alpha property as above.
what size to make and display your video is up to you. it will depend on its ratio; is it HD ? the rule of thumb is to try to play it at its native size ( the size it was encoded at), or scale it down from that.