The answer on largest range will vary due to type of chainrings, spacing between the two, derailleur, and chain, but I believe 48/28 is impractical due to the steep, low angle climb and drop. The derailleur pushes the chain sideways, not up, and the chainwheels are designed with certain assumptions. The derailleur will not in my opinion be able to push the chain that high, and in the other direction the chain will likely travel right past the small chainwheel due to the steep drop. Other large differences present similar problems. What you will get here is anecdotal evidence that may not apply to your equipment, as there is no definitive answer.

As to the smallest difference there is also not a single answer. With today's 11 and 12 tooth small cogs I don't think 53 is very useful, let alone paired with a 48. How often do you expect to be going 40+ mph when coasting is not the better alternative?