How large was Yoshimoto's army on his fatal campaign? The standard 45,000? The answer is no. The 45,000 strong comes from both Ota Gyuichi's Shincho-Ko ki and Oze Hoan Shinchoki. The two Nobunaga biographies have listed 45,000. However, the Imagawa army was smaller than originally thought.
The Hojo Go dai ki has the Imagawa army as 25,000 and the Koyo Gunkan (a chronicle of the Takeda) noted 20,000. I must admit I do not have the the Hojo nor the Takeda chronicles in my personal library. That being said, Owada Tetsuo's Okehazama no Tatakai did note down the two chronicles (pp. 22-25).
Kajino Wataru has an excellent map in his book, Jimoto no Karo ga Kataru Okehazama Kassen Shimatsu ki, p. 69. The map has a layout on where both armies were placed and their movements. Kajino listed the Imagawa army as 21,500 and Nobunaga's 4,300 (numbers are approximate). Okada Masahito's Oda Nobunaga Sogo Jiten noted down the Imagawa and the Oda 2,000 (p. 319).
As for the Imagawa main camp when conflict started, the general consensus was around (Yoshimoto) 5,000 and Nobunaga's 2,000. Once Nobunaga received the intelligence on Yoshimoto's whereabouts, it was divide and conqueror to make the odds of winning better.
In my opinion, the Imagawa army was around 25,000 during the campaign and the Oda 4,000 or less. As for the two armies during combat action, Yoshimoto's main camp 5,000 and Nobunaga's 2,000. Again, the numbers are approximate.
Tenka no tame!