Technically, you guys are talking about army corps, not armies, as this game is based at the strategic not tactical level, as an "army" at the strategic level would have 60,000 to 100,000 men,
http://www.feldgrau.com/org.html, just nickpicking there. I usually place my army corps at a size of maybe 6 divisions, one being an HQ, for "good" commanders that need training, ie high skill Lt. Generals who need more than 3 divisions to command, with more divisions under their command I can use them to spearhead offensives against gaps in the enemy line and get them valuable experience from
leading attacks rather than just participating. This gives me enough experience for those leaders that I can promote them quickly and get to work on the leaders with less skill, whom I give army corps of 3 until I can get them promoted to a general, panzer leaders get 2 armored and 1 mot. or mech. division most of the time.
Actual army sizes may vary in relation to the size of the entire land force. The Romanian third and fourth armies and the German Sixth armies can make good examples of this.
Romanian Armies:
3rd Army organization
1st Army Corps
7th Infantry Division
11th Infantry Division
2nd Army Corps
9th Infantry Division
14th Infantry Division
4th Army Corps
13th Infantry Division
1st Cavalry Division
"Colonel Voicu" Detachment (since 11 November)*
5th Army Corps
5th Infantry Division
6th Infantry Division
3rd Army reserve
15th Infantry Division
7th Cavalry Division
Heavy artillery
2nd Heavy Artillery Regiment
4th Heavy Artillery Regiment
5th Heavy Artillery Regiment
8th Heavy Artillery Regiment
41st independent motorized Heavy Artillery Battalion
*) "Colonel Voicu" Detachment
12th Infantry Regiment
13th Light Infantry Battalion
611th Panzerjaeger Battalion (German)
54th Pioneer Battalion
3rd Artillery Battalion (of 41st Artillery Regiment - 13th Infantry Division)
1st Artillery Battalion (of 4th Heavy Artillery Regiment)
4th Army organization
6th Army Corps
1st Infantry Division
2nd Infantry Division
18th Infantry Division
20th Infantry Divsion
1st Heavy Artillery Regiment
47th Heavy Artillery Battalion
in reserve:
6th Cavalry Regiment (5th Cavalry Division)
27th Pioneers Battalion
7th Army Corps
4th Infantry Division
5th Cavary Division
8th Cavalry Division
7th Heavy Artillery Regiment
in reserve:
57th Reconnaissance Group
57th Pioneers Battalion
http://forum.axishistory.com/viewtopic.php?f=4&t=53143
That places the Third Army at around 10 divisions and the Fourth Army at around 7 divisions. The Third was mostly put in corps of 2 divisions while the Fourth had corps of 4 and 3. The German Sixth Army had 250,000 men, which in HOI would come out to around 25 divisions, as they had a much larger land-based military overall, they could form much larger army formations without losing combat effectiveness. I can get a makeup of the Sixth Army as I have the Romanian armies above if it's required and you want to see how they organized their corps into army groups.
Overall, the choice on how to organize your military force is up to the amount of men and competent commanders available, and your personal choices on how you want them organized, so there is my worthless two cents.