I seem to remember before the Iraq war. A general named Yamazaki (it was something along those lines) said that at least 250000 ground troops would be needed to control iraq. Soon after he got fired and replaced with someone who advised only 100000 were needed.
That might help.
I also remember this firing. He was Chief of Staff of USA army, Scheski (sic). I remember him say at least 300,000 occupation troops would be needed.
Choices for Bush (Best to Worse):
1) Don't start war.
2) Invade Iraq with OTL forces. We then kill off Sadam, and give power to whichever Iraqi General will be pro-USA. Leave country within 12 months.
3) Invade with 300,000 to 500,000 troops. Then we will have the forces necessary to "nation build" and "create democracy". Even with the full USA army, and all the reserves, the USA army was too small. Assuming only 1/3 of USA troops in Iraq, we will need a full time Army of about 1.5 million. Rough rule of thumb is 1/3 of time training, 1/3 of time at front, and 1/3 of time in administrative status. In Korea, we kept one Brigade at front, one in unit training, one was in administrative status. Over a 3 year period, a soldier will have 3 months of leave, and there is a need for individual training schools for soldiers, and overhead.
4) OTL.