How can I get the horizon or vertical length of the uneven object?

Park the nose against a wall and measure, or have a measuring stick (long ruler 6ft or 2m) from the base of the windscreen with a drop line to find the bumper edge.

Use a masons' 6ft aluminum level with a plumb bob.

Drop the plumb line to any edge you want to measure from a point on horizontal level while the end of the level is at the end of the object.

measure the level from plumb line to the end.

Park the car on level ground and use a spirit level and a couple of tape measures.

Find straight ground. Doesn't have to be totally level. Fasten a straight and sturdy beam to something so that it's slightly higher than the hood, parallel to the ground (by measuring the distance in two places). Park the car under it. If you want to be safe, fix the beam to something that will give way, no need to accidentally smash the windshield. Clamp two pieces of sturdy cardboard or wood to the beam so they touch/describe the points you want to measure. Measure perpendicular to the beam by using anything that has a right angle, a ruler and measuring tape.

By using more than 2 pieces, you can even get the entire curve, if that's needed.

It looks like this is a model car, so you could use a granite table, and a height gauge for the vertical dimension.

For the horizontal dimension I would place a 1x2x3 block or similar against the front and measure from the face of that to the lip of the hood near the windshield using calipers. I would then subtract the thickness of the block.

Edit: Also if you know somebody with a mill that has a DRO, you could use that as a 3D measurement tool.