Re: Size of boat in relation to dock size....?
In most instances the dock and associated pilings have to serve two functions: landing and mooring. Typically you want to do both with the same structure.
first factor is water depth at the end of the dock. Two feet at low water can handle a 21' boat with an OB and a skilled operator. But let's assume there's 4' at low water so that's not an issue.
Second factor is the current conditions, including waves and wakes, tide height and the fetch of prevailing winds. This is important for two reasons. Strong conditions put more pull on the pilings, requiring different lay-outs as well as material size. Wave conditions require the boat to be pulled away from the dock and pilings (whips are worthless, and a boat should never touch any of the dock structure, even dolphins, when moored). If your water is 100% calm you can do with a lot less.
As a practical matter storm conditions are irrelevant for small boats because you will pull it out. As for the pier itself, it's a question of wave threat, going back to the fetch. Build it low, double the bents and shorten the stringer runs and you can survive more. However, in a summer squall, you can have short-term wave issues, and again, the boat has to be moored with real mooring lines so it cannot touch any pier structures. My calm cove where I moor my boat year round will get white caps and 3' chop in a squall.
Some places you have to factor in ice issues as well. If you have a lift you need enough depth to sink the whole thing without touching bottom (which will spring your cables).
Here's the solution to the short dock: outboard mooring pilings, including the use of pulleys. A weight/pully system gives you lots of options, and the weight acts as a nice shock absorber in wave conditions. A ten foot dock with additional pilings (dolphins) 10-15' away from each corner in line effectively extend the dock 20-30'. And you have to have pilings out away from the dock to pull the boat away from the dock.
At my house growing up we didn't have a pier so we used the bulkhead (sea wall as you call it) for landing and a piling/wire/pulley system for mooring. My parents built a 40' pier and we still used the pilings/pulley for mooring. I can easily moor 3 small boats (15-23) at the pier now with the pilings, shore posts, etc. and have had 6 boats there at a time short term, such as when we have a yard party.