The best size for a basic front-line formation is probably 20k or 25k tons, mainly due to the need for a lot of commanders. If you want to use multiple HQ levels (not required, but good for flavor and to use the support weapons), each HQ formation should be the same size and have a large enough HQ to control about 3 sub-formations. For example, if you consider 25k ton Brigades as your base unit, then a "Division" formation would have 25k tons (of support weapons, supplies, etc.) and a HQ able to control 100k tons, and you would attach 3x Brigades to that Division. A Corps could then control three Divisions (HQ size 325k tons), and an Army formation might control a million tons (3 Corps).
Usually to conquer an NPR home world you will need several million tons of ground forces at a minimum, possibly even in excess of 10 million in some cases. This requires building up a large number of ground force training centers and researching the ground forces training techs ideally.
A reasonable kind of line infantry formation might look like:
1x STA+HQ25
3000x INF+PW
300x INF+CAP
150x INF+LAV
100x INF+LB
186x INF+LOG-S
This is a reasonable 25k ton infantry brigade (actually 24,997 tons) with a good mix of support weapons but a focus on killing enemy infantry first and foremost. It is important to have a high proportion of basic rifles (PW component) so that combat losses will not mount up too quickly among the heavier support weapons. If you want to use higher HQ formations, the key components to use are bombardment (LB and MB are fine) and if you research Construction Equipment it is good to include some of these as well to improve your fortification levels. Note that NPRs do not use ground support fighters so the AA components are unnecessary unless for roleplay or use in multiple-player-race games.
It is also important to consider your logistics solutions, simply put to mount a successful planetary invasion you will need a lot of extra supplies. The best way to do this right now is to make formations with nothing but INF+LOG-S units, place these in the rear echelon, and set to "Use As Replacements" with the checkbox in the main ground forces window. These will replace the organic LOG-S in your front line formations as they are consumed (or, occasionally, suffer combat losses).
Finally, consider how you will get your multiple millions of tons of ground units into the battle zone. You need transports, especially drop-capable transports, and you also will probably need to deal with STO weapons on the ground - either by nuking them from orbit, or by using heavily-armored (8-12 layers of armor at least) drop-capable transports to absorb the ground fire and deploy your troops as quickly as possible. Either way, you need to build a lot of transports, so get started early.
www.abc.net.au
www.rottentomatoes.com
