How to split rent when rooms aren't the same size
Start with an equal split, then list meaningful advantages: private bath, larger area, light, closet, quiet, parking, or direct outdoor access. Price the features that affect daily life, not every inch of wall.
Use a simple blind bid if you disagree. Each person states what they would pay for each room; choose the combination that covers the rent and feels least unfair. Put the final rent and any included utilities in writing.
Revisit only when the layout or household changes. A clear split is better than a monthly debate.