It is difficult to put an exact measurement on a custom laundry room as it is totally dependent on your available space. The main factor to consider is whether you have enough room to fit in a washing machine and tumble dryer. Then you can begin to think about other considerations such as; an area for ironing, space for drying clothes, storage and possibly even a sink.
With a front-loading washing machine and tumble dryer, you will be able to stack them on top of each other, creating additional room. Alternatively, shelving can be placed above the two appliances if they are stood side by side, making front-loaders a much better option for a designated laundry room which is constrained by size.
Smaller laundry rooms can still have all the amenities and features of a larger room but it may require a little more planning and ingenuity.