Most one month olds should still be taking about 2oz every 2 hours. However, if your baby is low weight, has acid reflux, or other health issues, you should definitely speak with your pediatrician!
For babies under six months of age, a general guideline is two to two-and-a-half ounces per pound of body weight, per day. Infant feeding patterns vary widely, so some days they eat more, others less.
Try a few ounces of formula for a one month old. Watch for spitting up to indicate over feeding. Ten to sixteen ounce may be enough for this age. This is for an average birth rate of seven pounds.