How do I code for an .mp4?

     I have 3 1920 x 1080 HD videos

1 = 166mb

2 = 209.mb

3 = 157.mb

I have to compile all three to Premiere then add some graphics and effects so my question is the following, I need to have the final output in .mp4 so do I

convert the large files down to a managable size while retaining the ratio?

to  960 x 540

THEN import those files to Premiere, add the graphics and edits and then output to mp4?

If so what codec should I use from the original 1920 x 1080 to 960 x 540?

and then from 960 x 540 to mp4?

anyone?

r