All other things being equal, demesne levies are not affected by realm size. But they are affected a lot by other things, like: your personal martial stat; whether your marshal is training troops; province modifiers (eg. the +25% from prosperity); etc.Is levy size tied to how big the realm are?
Right now as an Emperor with total holding in my realm between 300-350 and have Great Wall in my capital, I can muster 25K levy from 5 castle in my capital (5K/castle)
If I just small King of Aragon/Sweden who has no plan to expand, can I still muster 25K from my capital if I have Great Wall?
It's also worth noting that retinue size is directly affected by realm size - a larger realm means more retinues. This is important because retinues can be a large fraction of your total armies - and they're also usually the best quality.