Local Militias
The smallest unit that can or should be formed by local militias should be roughly equivalent to a platoon - typically 30 to 50 volunteers, with appropriate arms (which should be hopefully standardized on a caliber, if nothing else).
Three to four platoons, from the same general region, should form a company; with the officer commanding being selected from the commanding officers of the platoons. Strength should range from 120-200 militia.
Three to four companies forms a battalion (480-800 militia) and three to four battalions a "column" or "brigade" (2,000 - 3,200 militia).
The local militia's job is to provide a modicum of military experience for most people (i.e. drill one weekend a month, provide upkeep on their weapons, etc), provide local security and policing in the event of war (civil defense and that sort of thing) and to augment the reserves and the regular army in defense, if necessary. They are incorporated into the national command structure and are de facto federalized (with the understanding that, unless in extreme cases or in case of volunteers, they will not generally be required to campaign outside their home region), although they can be called up by and commanded by local authorities in the event of emergencies or natural disasters. Unlike the National Guard, they don't go through any kind of boot-camp, but they are provided basic weapon training and some basic drill (ideally during their monthly training sessions).
The regulars should be called (appropriately enough, I think) the Continental Army, which will be organized along more conventional means. Their size is going to heavily depend on population and our starting situation, but should be mainly volunteers, at least at this point. They answer to our central government and are strictly suborned to civilian control. They are, of course, full-time soldiers.
As an auxiliary to the regulars, there's the National Guard, which are more of a reserve system and are there to provide a more ready pool of mobilizing manpower in the case of war and to help fill out the Continental Army's offensive power. They're not strictly full-time soldiers, but they train more often and have more "range time" than the militia (maybe something similar to OTL/present-day National Guard training, where they go through a basic training and then a period of military training before going to a one weekend a month/two weeks a year thing). It's therefore more of a commitment than the militia, although with therefore more compensation and potentially benefits. Also, ideally, volunteer.
So we have a three-tier mobilization system that gives us some flexibility in how much of our armed power we deploy and should ideally give us a pool of trained man- and woman-power to draw on in the event of a larger conflict.
Rather than a navy, I think we should have a Coast Guard to provide security, border patrol, law enforcement, etc, on Lake Michigan/the rest of the Great Lakes and potentially along the Mississippi. They're sort of a combination of military/LE, like the present day Coast Guard.
