What you're looking for is called a "mockup" or "mock."
It's very common to make a prototype of a garment from cheap fabric first, then test the fit and, if everything's alright, repeat the process with the expensive fabric. The mockup fabric should be at least similar to the final fabric in terms of stretchiness and heaviness.
Usually the mockup is the same size of the finished garment, because the whole idea behind making a mockup is to test how well it fits. If you want this to be a surprise and not test the fit, you could do a smaller mockup.
When you tested the size and found any problem, you adjust the mockup until it fits. You can sew seams tighter if there is bulkiness or you can piece on another strip of fabric if it's too tight. Once the mockup fits, you mark all the seams on the fabric. Then you pick it apart again and adapt your pattern to the same seams you marked on the mockup. That way you make sure the final garment will fit well.
People usually use cotton muslin or leftover fabrics that are sold cheap for their mockups. You can often find fabrics with Christmas prints or children's patterns for a reduced price in online shops. You can also use old bedclothes or tablecloths if you don't want them anymore, or have a look at second hand shops that might sell tablecloths or curtains for cheap.