Either development or gear-inches takes wheel size into account. Lowest gears can be 52 gear inches for really strong riders. Hills might make 45 gear inches a better choice. If you aren't strong up the hills, you may find 39 gear inches to your liking. Tandem riders may find a lowest gear in the high twenties to be a safe hill climbing gear.
Tandems often have triple chainrings so that really low gears are available.
The gaps between gears should be closest around your fastest cruising speed on level ground. You can have a bit of a larger gap between the low hill climbing gears. Top gear needs to be 110 gear inches or more if you want to stay with fast packs or tandems on the long downhills. When there's a big downhill, you rarely have a subtle transition to it. The freewheel at the back is where most riders make gear adjustments for a particular application.