Low Altitude is when the map is parallel to the planet's surface, and encompasses Altitudes 1-10. High Altitude is when your map is perpendicular to the surface, one hexrow is the ground, you have a few hexrows of atmosphere, and then space. The lowest non-ground hexrow on the High Altitude map encompasses all of Low Altitude. Low Altitude hexes are 500 meters across, High Altitude hexes are 18 kilometers across. Combat at Low Altitude uses ten-second turns, High Altitude and Space operations use sixty-second turns. Between those figures and Total War, all your velocity questions are easily answered.
The rules for all of this can be found in Total War. Low Altitude is on pages 80-88, and High Altitude/Space Movement is on pages 76-80. It is not possible for text to convey sufficiently strongly that you stay the hell away from the Aerospace Units on Ground Mapsheets rules on pages 91-92. Seriously, they will only bring you pain.