Changing export settings to 1080/60

Hi Mac people!

I recently shot some footage using a high-end DSLR camera and a GoPro Hero 4. The source settings on both were 1080/60fps. The raw footage is excellent quality. I imported to iMovie, via Photos and edited into a 5 min vid as normal.

However, when I come to export, it doesn't give me the option to export to the full 1080/60 settings, defaulting to 720/60. The full 1080/60 option is greyed out and there's nowhere obvious that I can go to change this.

Some investigation reveals that the first frame determines the size, The first frame of my movie is a title screen, followed by a 1080/60 clip. I can't believe that a title page would have the effect of rendering a whole movie lesser quality so would be grateful for any advice or pointer son this.

Thanks in advance!