Again, I ask. What's the goal?
That really is the question.
I see some overarching principles of determining the size and composition of an army (let's leave aside the air force and navy for the time being but basically the principles apply)
1) an army must be focused on war and not some extraneous civil service tasks which can be accomplished more cheaply some other way. The CAF has gotten involved in domestic disaster relief as a way of establishing our relevance to governments that saw it as already paid for labour;
2) the only relevant measure of an army's size is what part can be deployed or mobilized on operations in the way of units or formations;
3) to have any real merit those units and formations must be well equipped, trained, organized, and sustained; and
4) you cannot employ any formation that you cannot afford to procure, train, deploy and sustain. (Bill Owen "Euclid's Army")
Right now we're hard pressed to equip, train and sustain, some 25 battalion-sized RegF units. 100,000 PRes folks equates to roughly another 166 battalion-sized units (@600 per unit). Do we really need or want another fifty light infantry battalions? Or 20 light reconnaissance regiments? And please don't say we need a PRes home guard security battalion at each international airport. At roughly the pay envelope cost of 1/6th of a RegF battalion, 166 PRes units would be the rough equivalent in annual pay to doubling the size of the RegF number of units. And that's not counting the additional full-time burden of recruiting and training another 8-10% of the force (roughly 3 PRes battalions annually; 6 if you add in the RegF) to replace the annual turnover.
And then there is the capital and maintenance cost of equipment and the annual training costs including ammunition.
I can see a realistic need to expand the army to some 75 hybrid battalion-sized units. That would require growing the ARes to some 30,000 personnel (roughly an additional 10-12,000 and reclaiming all out-of-unit Class Bs) and would provide an adequate number of ready forces as well as a base for even further mobilization and expansion. It's also an affordable amount for capital equipment and training costs tied to realistic national mission objectives. That would have a field force of some 20,000 RegF, 30,000 ARes and a BTL/ATL of roughly 5,000 RegF/ARes combined.
Want a home guard - figure our how to make better use of the Rangers and SuppRes.
Having been a young gunner in the ranks back then I can pretty much say that it's a given that a large number of them will be lacking in maturity when they join. It comes with time and being given responsibility and trust. Quite a few never make it. Most do.
