A meter. A kilogram is a unit of mass, not of length.
Length: meter Mass: kilogram Volume: cubic meter
I will measure a book with grams
no
The length of a parking lot is typically measured in yards because it provides a more reasonable and practical unit of measurement for larger distances. Measuring in inches would result in very large numbers that are not commonly used for such purposes.
you would use a kilogram
A kilogram is a measure of mass therefore a kilogram of lead and a kilogram of feathers have the same mass: one kilogram. They would, however, have different volumes and densities.
Potatoes are sold by the kilogram.
You would measure a rice plant (or perhaps the length of a rice field) in meters. You would generally measure a container of rice grains (for cooking or eating) in kilograms, unless you were concerned about how much space it would take up on a shelf or something like that.
Grams
it would be a gram and if you didn't know a kilogram is equal to 1,000 grams
Kilogram is a measure of weight - so you would use it to weight something. You would probably use it for objects that are large enough to make using grams unnecessary.
Neither is "largest" - a Kilogram is a measure of MASS, 1000 grams. Little over two pounds. A Meter is a measure of LENGTH, a little over a yard. Therefore, there is no way to say one is "larger" than the other. Unless you mean a square meter, and to find out if a kilogram or square meter of a substance is larger you would have to know the substance's density.