A day pack of about 40 litres is perfect to carry on the Machame route and each day will be a little bit different as to what goes inside it. Water bottles, rain gear, a few spare clothes, gloves and hat, sunscreen and lipsalve, snacks and camera equipment are all usual things to carry.
You may want an umbrella for showers and to keep off the sun on a still day. Trekking poles are also useful but you don't need to use them all the time. A flask for hot drinks is very useful on summit night, as is a headtorch. Also a rucksack cover or even a poncho to cover yourself and your bag. Some people bring mini spikes to put on their boots for summit night when it can be very slippy, but this is optional.
The weight of your day pack should not be more than about 5-6 kgs; as you go higher, the weight will seem heavier and harder to carry. Meanwhile your main duffle bag will be carried by the porters.