Schleswig-Holstein, in its present boundaries, covers an area of 15 700 sq.km.
The parts that were given to Denmark in 1920 were about 4 000 sq.km in size.
Comparison (1 sq.mile = 2.6 sq.km, 20 000 sq.km = 7 692 sq.miles):
Connecticut 13 000 sq.km, New Jersey 20 300 sq.km, Nebraska 200 000 sq.km.

For the year 1987, the census lists 2 550 000 persons as living in Schleswig-Holstein.

The Duchy Holstein has always been the part of Schleswig-Holstein that held a German population, whereas the Duchy Schleswig was always home for a mix of Danish and German inhabitants, and the further north one got, the more the majority swayed in favor of the Danish people.

Prussian - German: the Prussians were Germans, of course, but Germany as a nation (The "German Reich", "Deutsches Reich") came into existence only in 1871. Prussia was a kingdom then. One of several. In 1867, most of the places, areas, and islands named below were incorporated into the Kingdom Prussia, as the province "Schleswig-Holstein, Prussia".

Alsen, Als Island in the Baltic Sea, 316 sq.km. 1326/1435: Duchy Schleswig.
1867: Prussia.1871: Germany. 1920: Denmark.
Altona Free town in Holstein, in Danish times Denmark's southern capital.
1867: Prussia. 1871: Germany. 1937: integrated into Hamburg.
Amrum Island in the North Sea, 20 sq.km. Danish. 1864: Duchy Schleswig .
1867: Prussia. 1871: Germany.
Angeln, Anglia Roughly the area between the Baltic Sea coast and the towns Flensburg, Schleswig and Kappeln, to draw a line. Part of the former Duchy Schleswig. With a mixture of Danish and German population, formerly. Very rural area, and once a stronghold of serfdom. Also origin of the Anglians (Anglo-Saxons) who, in the fifth century, settled in England ( = "Angel-land").  
Ärröe, Aerø Island in the Baltic Sea, 86 sq.km. Duchy Schleswig. 1864: Denmark.  
Dithmarschen Part of the Duchy Holstein. 1 360 sq.km. Bordered by the North Sea coast in the west, the Elbe-River estuary in the south, the Eider-River in the north, and roughly the Kiel-Canal in the east. Major towns: Meldorf, Heide, Marne.  
Eiderstedt Peninsula stretching into the North Sea. Towns: Tönning and Garding  
Fehmarn Island in the Baltic Sea, 185 sq.km. 1435: Duchy Schleswig.
1867: Prussia. 1871: Germany.
Föhr Island in the North Sea, 82 sq.km. The western half was Danish until 1864 (Westerland-Föhr), the eastern half part of the Duchy Schleswig.
1864: Duchy Schleswig. 1867: Prussia. 1871: Germany
Geesthacht A town that was formerly part of Hamburg. 1937: Schleswig-Holstein.  
Helgoland, Heligoland Tiny island in the North Sea, less than 1 sq.km .
1358: Duchy Schleswig.1714: Danish. 1807: English. 1891: Germany. We swapped it for Zanzibar !
Lauenburg Duchy. Germany (1705 - 1803 Hannover). 1815: Duchy under the Danish Crown. 1867: Prussia. 1876: Schleswig-Holstein, Prussia, Germany  
Lübeck Free town and duchy. Germany. 1937 part of Schleswig-Holstein, Germany.  
Nordstrand Island in the North Sea, 48 sq.km. Duchy Schleswig.
1867: Prussia. 1871: Germany.
Pellworm Island in the North Sea, 36 sq.km. Duchy Schleswig. 1867: Prussia. 1871: Germany.  
The Probstei A small area east of Kiel, and 10 miles north of Preetz. Duchy Holstein.
Parishes: Schönberg and Probsteierhagen. Villages: Laboe, Stein, Wentorf, Barsbek, Wisch, Lutterbek, Brodersdorf, Prasdorf, Passade, Fiefbergen, Krokau, Stakendorf, Krummbek, Gödersdorf, Bendfeld, and Höhndorf.
Predominant names there: Schneekloth, Stoltenberg, Wiese, Lamp, Muhs, Arp, Lage, Finck, Göttsch, Sindt, Sinjen, Stuhr, Untiedt, Vöge, Ewoldt, Puck, Klindt, Stelck and Steffen, to name a few.
Many emigrants have their roots in this area.
Röm, Rømø Island in the North Sea, abt. 70 sq.km. The southern half was Danish until 1864, the northern half part of the Duchy Schleswig.1864: Duchy Schleswig. 1867: Prussia. 1871: Germany. 1920: Denmark.  
Sylt Island in the North Sea, 94 sq.km. The northern tip was Danish until 1864, the rest of it part of the Duchy Schleswig.
1864: Duchy Schleswig. 1867: Prussia. 1871: Germany.

Never part of Schleswig-Holstein, but in 1937, the following towns and villages were shifted from Schleswig-Holstein into the municipality of Hamburg: Altona, Wandsbek, Billstedt, and Rahlstedt. And: Ottensen, Bahrenfeld, Oevelgönne, Othmarschen, Blankenese, Gross Flottbek, Klein Flottbek, Lurup, Eidelstedt, Dockenhuden, Nienstedten, Osdorf, Rissen, Sülldorf, Langenfelde, Marienthal, Hinschenfelde, Jenfeld, Tonndorf-Lohe, Lokstedt, Schnelsen, Niendorf, Bergstedt, Oejendorf, Kirch Steinbek, Schiffbek, Duvenstedt, Bramfeld, Hummelsbüttel, Lemsahl-Mellingstedt, Lohbrügge, Boberg, Sande, Poppenbüttel, Meiendorf, Oldenfelde, Sasel, Steilshoop, and Wellingsbüttel. Some of these places had previously been incorporated into bigger ones, or they had merged with others, and they came to Hamburg as parts of bigger units