To give context, I'm making a story and in it there has been a civil war between people who want to establish a republic and fighting a corrupt monarchy. However, it has lasted for around 200 years meaning the population is tired and weary. The overall population of my fictional country is 10 million and the Monarchy controls around 6 Million of these.
The nation is very decentralized with local lords ruling over castles and cities.
//edit My story is based on the Hundred years war so even though the civil war has lasted 200 years there are many periods of truces and white peaces between periods of war. There is in fact enough grain and water to feed army provisions and both sides of the government have the money to create weapons and armor for the soldiers.
The general rules of thumb for the maximum population you can draft in your army in a medieval period are as follows:
No pre-industrial society can manage to put to the field more than seven percent of its entire population. This is your hard limit: no way you are surpassing this.
Due to the severe economic effects of mustering men for war, the amount of personnel you can take out of the economy is usually around a meager one percent of the country's population, possibly two. For a campaign of very short term(a couple months) you can risk more, especially in periods where the agricultural cycle means there are a lot of empty hands(such as shortly after the harvest).
That you have the men to muster does not mean you have the money to equip or pay them, or the food to afford to keep them in the field.
So, if there is an 6/4 million divide in your kingdom, the largest somewhat long term army size(that is, not for a short, one-month campaign) is 60-100 and 40-70 thousands respectively. This assumes both sides have enough food to afford to put that many men onto the field, and the gold to equip them: you have not told me much about the kingdom's situation so I can only ignore them to focus on manpower.
Of course, these forces will be deployed across many places, and battles will necessarily involve only portions of what you have the money to recruit. England's population was about five million by the Hundred Years War, yet it was a rare battle English troop count rose above 15.000.
On a side note, if 'it lasted 200 years' refers to the civil war I urge you to reconsider it. No war can reasonably last anything close to that period.