Q. Sir, the size of the garage is 15 feet long, 7.8 feet wide, what size car do I need?

Bro from garage size you should not go for a car. Your expectancy out of vehicle like looks, mileage, power, then type like hatchback or csuv or suv or sedan . Many factors are there please mention the budget the most important factor

The garage size is good enough for most cars. If you want an SUV, you can fit in upto something like a Creta / Audi Q2 ( easily ) but not a Harrier or Fortuner. Sedan wise Verna/Rapid will fit (tight fit though, just enough). Basically you can go for 4 meter plus cars. Just do not buy a D Segment ( greater than 4.5 meter long) vehicle.

Ho for any small hatchback or maybe compact SUVs. Those will fit. Make sure the car is no longer than 14 feet. Keep it in mind that you are not doing packing but parking a car.