Which is bigger in width mm, cm, m,km

Km is the biggest measure in width (even if there are bigger measures like Mm, Gm, Tm).

1 Km = 1000 m, 100000 cm and 1000000 mm

Hello!

This is kind of going to sound complicated...but here you Go!

Lets start off with MM:

The millimetre or millimeter is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length. Therefore there are thousand millimetres in a metre. There are ten millimetres in a centimetre.

2nd, CM:

A centimetre (international spelling as used by the International Bureau of Weights and Measures; symbol cm) or centimeter (American spelling) is a unit of length in the metric system, equal to one hundredth of a metre, centi being the SI prefix for a factor of 1100.

3rd, M: 

The basic unit of length in the metric system; it was originally planned so that the circumference of the Earth would be measured at about forty million meters. A meter is 39.37 inches. Today, the meter is defined to be the distance light travels in 1 / 299,792,458 seconds.

And Last, KM:

The kilometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: km; /ˈkɪləmiːtər/ or /kɪˈlɒmɪtər/) or kilometer (American spelling) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for1000).

So From all this information that i gave you, you might be thinking:
 
"Why on Earth Did you Give me ALL Of this when what i only needed is the ANSWER???" 

Well, i gave you all of this so you could have an IDEA of what all these measurements and how HUGE they can be BEFORE i Give you The "Answer". So as Final or Result, they Biggest Measurements from ALL of the Above would be: 

"KILOMETER"

1 km = 1000000 mm (Millimeters)

1 km = 100000 cm (Centimeters)

1 km = 1000 m (Meters)

Hope this ALL Helps! Took Me Time! Have A Wonderful Day! :)