Generally speaking, a longer rod has more spring properties to it as there is more surface area to bend. This can help you cast further as more bend equals extra stored energy, in turn helping sling your line forward. Also, when beach fishing the extra length helps keep your line above the breaking waves which in turn makes it much easier to feel and see bites on the line and rod tip. Around rocks, extra length helps in keeping your hooked fish away from the rock edge and snags and also makes retrieving your line easier in some cases when fishing off ledges and alike.
Biggest disadvantage to me is weight as often heavy due to the size and big beach rods 12ft-14ft+ can take a bit of casting practice. But like anything new, practice makes perfect.
The mostly used length of rod would have to be around the 6-7ft range. This size offers good cast length, all round useability and easy transport, especially if a 2-piece rod. On the shorter end, you have 5ft and under, a lot of boat/game rods used to be around this size, but trends have changed over the years and the rods used seem to have grown. Generally a shorter rod will be more stocky (stiff/fast action) and not have as much of a flex (soft/slow action) to it but great lifting/pulling power due to the low flex.
For just casting I like around a 7ft medium fast/ fast action rod with big guide eyes and a max of 6 as this reduces drag on the line helping cast further.
I like a 12ft rod off the beach and rocks.
For estuary/jetty fishing a nice 6ft-7ft rod.
If not casting, then 5-6ft from a boat is all that’s needed and saves on space.
When game fishing I like 6ft-7ft stiff fast action sticks as you get back more line in each lift compared to a shorter softer stick.
I hope this has helped. If you need any other information please just drop me a line.
The Sydney Marine Man