Rabbits need about 1-2 oz of dry food each day, per lb of body weight, including both pellets and hay. Forage(dry weight) can also be included in this, but not veggies.
The easiest way to figure the hay amount, is by the handful. A small breed rabbit(2-3 lbs) will need approximately one large handful of medium soft grass hay per day, at a minimum. So at least one large handful per 2-3 lbs of body weight per day.
Now this amount may need to be adjusted along with pellets, depending on the quality and coarseness of the hay. More pellets may be needed when a coarser hay is fed. Or feeding fresh forage, fresh grass, etc.
It's all about the right balance of protein and nutrient rich food for health and weight maintenance, along with enough indigestible fiber for good gut health. Indigestible fiber is what drives gut movement. Soft hay and leafy forage has more protein and nutrients and less indigestible fiber, coarse hay and coarse forage has mostly indigestible fiber and very little protein and nutrients.
Cuts of hay
The best way to monitor if you have the right balance in the diet, is by a rabbit maintaining a healthy body condition, as well as by the quality of their fecals balls. Large, goldenish, friable(crumbly) fecal balls are the ideal and show there is good gut motility.
Monitoring your rabbits weight
https://bunssb.org/bunnies/guide-bunny-poops/