There is no single size and type of rebar that you can use for all structures.
For small-sized projects, rebars below #3 are usually enough. For example, the bars used in driveways and patios are around 1/8 inch. It has to be kept in mind that this will bear minimum loads. It will also provide very little tensile strength.
When discussing walls, piers, or columns, rebars of more than #3 should be used.
Similarly, for footings and foundations, it is wise to use bars of size more than #4. Because the footings and foundations bear a lot of weight.
For large-sized projects, structures with longer spans require rebars of greater size i.e. more than #5. For example, the construction of bridges, tunnels, and canals. Some common structures are given below with the usual range of rebars used.
Concrete slabs: Contractors often use the 1/8 rule for determining the size of steel for a slab. For example in a slab of 6-inch depth, a rebar of 6/8 inch will be used. For slabs of septic tanks, you usually use #3 and #4 bars.
Retaining Walls: The most commonly used rebars in retaining walls are #3, #4 and #5. A 2-inch cover is usually provided for these.
Swimming Pool: The minimum rebar sized to be used for a residential swimming pool is #3. The maximum is #5.
Rebar Chair: These are supports to be given to rebars during placement. The size of rebar support chairs is less than the #3 bars.