To measure a doorway or opening for a gate:
With a tape measure, carefully measure the width of your opening from mounting surface to mounting surface. Note the measurement and refer to it when making your gate selection.
To measure the interior of your vehicle for a barrier:
With a tape measure, carefully measure the height of your vehicle, from floor to ceiling. Then measure the width from window to window. (Or the space that you would like to barrier.) Note the measurements and refer to them when making your barrier selection.