It is great to see you thinking about the experimental design here. No, you do not need to have the same amount of people in each family. In order for a result to be accepted as scientific evidence, it must be statistically significant. Basically what you must show is that it is unlikely that your results could have occurred randomly. The more families and family members you can examine, the more significant your results will be. I would suggest using families within a certain size range (like 4-6 people), but the key here will be to examine as many different families as you can. Also keep in mind that it is the children that share similar genetic information (DNA), so the more children in a family the easier you will be able to see a pattern.