Designing a small home takes skill on the part of the designer and extra work on your part. If you go through exercises such as these, you will be able to give your designer more information which will be helpful in the design process. Using these general concepts as a guide, continue with your own exercises to help determine the sizes your new rooms will need to be.

your kitchen space
In the kitchen, when space is at a premium, it's important to carefully analyze how you work, then think through how to organize the space in your new, smaller kitchen. You can do that by observing yourself in your current kitchen.
At our house, we do quite a bit of cooking and baking from scratch. This is our current "baking center." The dry ingredients for general baking are stored within arms reach, along with the measuring utensils and mixing bowls. On the left of this photo you'll see our small appliance garage; ideal for our heavy duty mixer. When pancakes or muffins are on the menu, ingredients can be assembled easily. This doesn't take much space; just thoughtful organizing. Think about the size and shape of the work surface you'll need; in this case, 3' of counter space is all we have. Florescent under-cabinet lights illuminate the work space without casting shadows like overhead lighting does.
How do you work in the kitchen? What size and shape is ideal for the way you work? What works in your kitchen now? What doesn't work?
Take a tape measure with you into your current kitchen as you actually prepare a meal. How much space to you really use in preparing that meal? What utility items do you use in your preparation?
Do you use 20% of your pots and pans 80% of the time? What can you get rid of, or at least store in the garage for those occasional events and what should be close at hand in the kitchen every day?
Pack up those extras and get them out of your kitchen. Do you miss them? If you can live a year without them, it's time to pass them on.

sizing spaces
Take your tape measure with you and measure the spaces you have now, in order to help determine the best places to trim in your new, downsized home. This photo shows an exercise I went through to help determine the size of our new porch. First I measured our current porches, then estimated the size of our new, smaller porch. Using some old flour, I marked the lines of the new porch so it became very visible. When I decided to enlarge or shrink an area, I simply swept off the old line and created a new one. I then tried various furniture arrangements in the new, smaller porch to see how they fit, also considering traffic flow.
In determining the sizes of your new spaces, invent ways to make the invisible visible, such as marking the ground with flour or the floor with masking tape. Live with it for a while and see what you think. Keep track of the measurements so you can let your designer know the results.

These same exercises can be applied to every room in your home. Think about what the room is used for and what you need in order for the space to work for you. In doing this you'll see where you can trim and how to make the space more efficient for the purpose.