First never break a truce as a beginning player, especially in the HRE even an experienced player can get his game ruined by a massive coalition. Later on there might be situations were you know you can break a truce, because you can take the penalties, but as a starter I wouldn't do it untill you know the game good enough to know exactly what you're doing.
Your army size should always be under your forcelimit in my opinion. There are people that build up until the limit, which can when you are a larger country with lots of manpower reserve and income or when you decide to go on a rampage and take out a few minors the next 10 years, so you know you'll be using them fulltime. Otherwise I usually keep it up between 2/3 and 3/4 of the forcelimit. I usually play minors though with less manpower reserves. This way you save money if you have years of peace, I always keep a standing army though, because the AI seems to calculate your army size to decide if it's going to attack you or not, don't know this for sure, but from my experience it does. Also you have a limit on mercenary's and when war is declared on me I usually train mercenaries to fill up my army to forcelimit or even higher if needed. You could also build up to max and decrease the army maintenance, I never use this, simply because no mather how often I've made the mistake, I still forget to increase it when I go to war and get stackwiped because of the low moral. If you are the player that never forgets these things it's a very interesting option, for me it just doesn't work and with a good economy it isn't needed.
You can find the forcelimit in the topleft, when you click on the icon under your countries flag. There you can quickly train troops, ships, buildings, coring, development etc. If you open it your on the army tab and at the bottom you'll see your troops and your army limit, for example 23 / 40. If you go above your forcelimit you'll pay more upkeep for those troops.
For the warscore you get 100% when you've completely occupied all your enemies. Usually you can seperate peace out some of their allies, so your warscore gets higher. Also you don't need to have 100% perse, usually if their war enthousiasme is low they will be willing to even except a peace with a higher cost then your warscore and often if you don't need that much it isn't needed. The cost of an ongoing war can be higher and taking one province less and take that in a later war is often a better choice. Also in my experience an ongoing war can be very heavy if your economy is ruined and you don't have any manpower left. Also look at your own allies, if they are tired of the war they can make a seperate peace. The AI usually takes way more loans and can go all in on a ongoing war, while I usually hold back on loand untill I discover they have recovered pretty well and it's actually to late. Especially against France it's often better to take two provinces when you're ahead, because they can have a very long breath and are able to recover very well with their huge economy and manpower. If you are ahead and your country is pretty tired because of the war it's better to peace out and recover first. For Granada I would peace out Tunis and Marroco, usually their allies and then you have a 100% warscore if they are fully occupied. Another option is that you don't get a very long truce if you don't take much, so you could just force them to pay you and to end their alliances. They probably won't get a new ally, perhaps themclen (or something like that) and there isn't a really big change that they get conquered by anyone else. So then you can take them pretty easily and with low costs after the truce ends, which would be within the 10 years that they need to break their alliance with Tunis and Marroco.
Also the wargoal is important, if you occupy the wargoal or win enough battles if thats the wargoal, you'll get a ticking warscore that goes up to 25. When defending keeping the AI from taking the wargoal can sometimes mean that you can eventually white peace out even when losing most of the battles and part of your country besieged.
Breaking up armies is very usefull when you have a large army. A province can support a certain kind of troops, if you have more you get attrition, so later on walking arround with a 60 troop doomstack will decimate your manpower and troops. Always look out for the little death skull symbol to the right of your troopnumber, which indicates that your troops a suffering attrition. When split up you do need to keep your troops together when fighting a strong opponent, so that when you are attacked you can pile on the enemy from all sides and reinforce your defending troops. To minimalize attrition losses with sieges you can detach the troops needed to succesfully siege the province and pull back the rest in the province you own or occupy. This way they can always quickly reinforce, but you have lesser attrition. It can sometimes be better to siege with your whole stack though, because a larger army makes the siege go faster. Especially if you and your ally are attacking a large nation on different fronts, for instance when you would attack France with Burgundy, you would want to siege some critical forts as soon as possible, so you and your ally can reinforce eachother, which ways up against the extra attrition you might take.With enough manpower I would sometimes even storm the fort when there is a breach in the walls.
