So the armies in size order might be:
Russia
Germany
France
Austria
Turkey?
Britain/Empire?
Italy?
I don't know the exact size of the Russian Army in 1914, but it was very large........
THE RUSSIAN ARMY, 1914
The German army in 1914 consisted of 87 divisions with about 2.3 million men.
This is a link to the Austrian OB in 1914
Austro-Hungarian Army - Armed Forces 1914
In 1914 the French army consisted of 1.7 million men, organized into basically 62 continental divs. There were approximately 15 additional divisions deployed overseas and not counted in these manning levels
In 1914 there were 700000 men in the british army incl reserves, organized into 6 regular and 14 territorial divs. I'm not entirely sure of the empire strength, but the Anzacs amounted to 4 divs expanding to 7 divs during the war (incl the mounted div)
Before the Ottoman Empire entered the war, the four armies divided their forces into corps and divisions such that each division had three infantry regiments and an artillery regiment. Although I am not 100% sure, the ottomans fielded an army of approximately 1 million men. The main units were: 1st army with fifteen divisions; 2nd army with 4 divisions plus an independent infantry division with three infantry regiments and an artillery brigade. The second army headquarters was located in Syria commanding two corps made up of two divisions. 3rd Army with nine divisions, four independent infantry regiments and four independent cavalry regiments (tribal units); and the 4th Army with four divisions. The Redif system had been done away with, and the plan was to have reserve soldiers fill out active units rather than constitute separate units. In August 1914, of 36 infantry divisions organized, fourteen were established from scratch and were essentially new divisions. In a very short time, eight of these newly recruited divisions went through major redeployment.
By November 1914, the Second Army was moved to Istanbul and commanded the V and VI Corps, each composed of three divisions. The Ottoman concentration plan shifted major forces to European Thrace and established the defense of straits. The First and Second army located in this region. The Third army acquired new supplies for a winter offense. The force in Palestine (VIII Corps) is replaced with the Army in Mesopotamia.
The Italian Government introduced military conscription in 1907. However, only about 25 per cent of those eligible for conscription received training. By 1912 there were 300,000 in the Italian Army but critics point out that there was a shortage of experienced NCOs and trained officers. The performance of the army suffered considerably because of those shortages.
In July 1914 General Luigi Cadorna became chief of staff of the Italian Army. Although the Italian government declared its intentions to be neutral on the outbreak of the First World War in August 1914, Cadorna expected war and began building up his army. He decided to concentrate his forces on the borders with Austria-Hungary.
At a secret meeting held in England on 26th April 1915, representatives of the Italian government agreed to enter the war in return for financial help and the granting of land currently under the control of Austria-Hungary. The Treaty of London resulted in Britain granting an immediate loan of £50 million and a promise to support Italian territorial demands after the war. Without that help it is reasonable to assume that the Italian Army would have found further expansion very difficult. I cannot see the Austrians or the germans being able to match that financial underwriting.
By the spring of 1915 General Luigi Cadorna had 25 infantry and 4 cavalry divisions (by comparison, the British army by then consisted of over 50 divisions plus about another 20 colonial divs) . Grouped into four armies, Cadorna only had 120 heavy or medium artillery pieces and some 700 machine guns. Despite the shortage of artillery Cadorna launched mass attacks on Austria-Hungary in May 1915. The defending army quickly built trenches and the Italians suffered heavy casualties. In the first two weeks of the Isonzo Offensive, the Italian Army lost 60,000 men. By the time the attacks were called off that winter, Italian casualties had reached 300,000. Cadorna had also lost 3,000 field guns, which had mostly been supplied by Britain.
Cadorna made seven attempts to break through enemy lines but his only substantial victory was at Gorizia in August 1916. The Austro-German Caporetto Offensive in October 1917 was disastrous for the Italian Army. Over 300,000 men and most of its trench artillery was lost and the following month General Luigi Cadorna was replaced by General Armando Diaz.
Diaz managed to stabilize the front-line but was unwilling to undertake an offensive of his own. Vittorio Orlando, the Italian prime minister, was aware that he would need military victories to strengthen his negotiating position to obtain territorial demands after the war. He therefore pressurized General Armando Diaz to launch an offensive in a war that would soon be over.
On the 23rd October 1918, Armando Diaz launched an offensive at Vittorio Veneto. Diaz was able to put 57 Italian divisions in the field. These were supported by soldiers from the French Army and the British Army.
After initial difficulties crossing the Piave River, the Italian Army took the town of Vittorio Veneto. The Austro-Hungarian Army collapsed and by the Armistice the Allied forces had reached Trento in the west and Tagliamento in the east. When the ceasefire was called on 2nd November, the Italians had taken 300,000 prisoners.
Italy's total wartime casualties was 420,000 killed and almost 955,000 wounded out of the 5.2 million men who served during the First World War.