It's kind of a non-issue, is it not? An AI can be projected at whatever size and resolution available via holoemitters. When on the Autumn, Cortana was about 2ft tall. When she was loaded into Alpha Halo's control room, she was much larger than the Chief. When she appeared at the end of Halo 4, she assumed a size comparable to the Chief.

Sloan can be any size, right?

I can't tell you definitively how large he was being projected at Meridian Station as I don't have Halo 5 3d assets.