Ultimately the decision is based mostly on what you like and your budget. We do have some important recommendation based on 11 years of research!
If you live in a cul-de-sac or dead end road, a one sided sign would work just fine for you. In this situation, your sign needs to be at least 20"-30" tall with a minimum of 4" reflective numbers. In this situation traffic is usually slowing or already slow with plenty of time to see your sign.
If you live in a community where the speed limit is 20mph - 35mph you may want to consider a two sided sign at least 36"- 48" tall with number 6" or larger. Keep in mind the layout of your property. If you are on a hill or rise you can go with a shorter sign. If the property lies lower than the road you may need a taller sign to get the desired effect.
If you live on a major road or highway with speed limits of 45mph or higher, your numbers may need to be larger. If a lot of big trucks go by, you may need to consider a heavier plate or a fixed plate design.