The 10 smallest states of Nigeria by land area are:

Lagos
Anambra
Abia
Imo
Ekiti
Ebonyi
Akwa Ibom
Enugu
Federal Capital Territory
Osun

Full list of Nigeria states by land area. From smallest to largest

State	Area (Km²)
Lagos	3,475
Anambra	4,761
Abia	4,857
Imo	5,135
Ekiti	5,797
Ebonyi	6,342
Akwa Ibom	6,788
Enugu	7,560
Federal Capital Territory	7,569
Osun	8,585
Bayelsa	9,363
Rivers	10,361
Ondo	15,019
Ogun	16,850
Delta	17,095
Gombe	17,428
Edo	19,584
Kano	20,389
Cross River	22,112
Jigawa	23,415
Katsina	23,822
Plateau	26,539
Nasarawa	26,633
Oyo	27,036
Kogi	29,063
Benue	30,755
Sokoto	32,146
Zamfara	33,667
Kwara	33,792
Kebbi	36,320
Adamawa	37,957
Kaduna	44,217
Yobe	44,880
Bauchi	48,197
Taraba	59,180
Niger	72,065
Borno	72,767
Total	911,519

By the time that Nigeria got independence it was a federation of the three regions, namely:

Western
Eastern
Northern

Later on another region was added, the Mid-Western Region.

The Mid-Western Region was formed from the Western Region in 1966, and Lagos, the capital, was effectively governed as an unofficial fourth region outside the bounds of the Western Region.

This brought the number of regions to 4 as follows:

Western
Mid-Western
Western
Northern

Under the short-lived military government of Aguiyi-Ironsi, Nigeria was reorganized under a central government. Following a coup which resulted in Aguiyi-Ironsi’s deposition and assassination, Nigeria was reorganized as a federal country again with three of the regions being divided into newer entities and all first-level subdivisions being renamed as states.

Western Region was divided into:
Lagos
and Western states.

Eastern Region was divided into:
East-Central,
Rivers,
and South-Eastern states;

Northern Region was divided into:
Benue-Plateau,
Kano,
Kwara,
North-Central,
North-Eastern,
and North-Western (Sokoto) states

In 1976, six years after the end of the civil war, the states were further reorganized:

State boundaries and names were also reorganized.

Benue-Plateau state divided into:
Benue
and Plateau states.

East-Central state divided into:
Anambra
and Imo states.
Federal Capital Territory (Abuja) formed from parts of Niger and Plateau states.

North-Eastern state divided into:
Bauchi,
Borno,
and Gongola states.
Niger (Minna) state split from Sokoto.

Western state divided into:
Ogun,
Ondo,
and Oyo states

Akwa Ibom state created from Cross River;
Katsina state created from Kaduna

Abia State created from Imo;
Bendel State divided into Delta and Edo;
Enugu State created from Anambra;
Gongola State divided into Adamawa and Taraba;
Jigawa State created from Kano;
Kebbi State created from Sokoto;
Kogi State formed from parts of Benue and Kwara;
Osun State created from Oyo;
Yobe State created from Borno.

Bayelsa State created from Rivers;
Ebonyi State was formed from parts of Abia and Enugu;
Ekiti State created from Ondo;
Gombe State created from Bauchi;
Nasarawa State created from Plateau;
Zamfara State was created from Sokoto.