Does my height above the ground affect whether I get frozen by Mei's ult (Blizzard)?
Solution 1:
It's right there in the Wiki-ish. The range is 8 meters radius. Meaning you'll ger frozen if you're in direct LOS of 8 meters or less of Snowball. Similarly Reaper has 8-meters ultimate radius as discussed here
Solution 2:
I've recently learned and tested that Mei's ult is not a circle but actually a sphere. If you are directly above her ult but hovering a few feet off the ground you will freeze. If you were the same height but closed to the edge you are less likely to freeze. The visual for this isn't shown very well.